fix: update Gas Services docset (#689)

Co-authored-by: Stephen Fluin <stephen.fluin@gmail.com>

src/pages/dev/general-message-passing/gas-services/increase-gas.mdx

@@ -1,16 +1,20 @@
-# Increase gas
+# Increase Gas
 
-On occasion, the prepaid gas to the Gas Receiver contract could be insufficient, such as when the destination chain is congested with many transfers or due to other conditions. Therefore, Axelar provides an option to resubmit a new amount of gas, as well as an option to refund the paid gas. 
+On occasion the prepaid gas to the Gas Receiver contract could be insufficient, such as when the destination chain is congested with transfers. When this happens, the application can resubmit a new amount of gas to pay for the transaction through [Axelarscan](http://axelarscan.io), the [AxelarJS SDK](https://docs.axelar.dev/dev/axelarjs-sdk/intro), or via a direct invocation of the [`AxelarGasService` contract](https://github.com/axelarnetwork/axelar-gmp-sdk-solidity/blob/ad37802dc6d62fff3ab589f2605f7a3e566977dd/contracts/interfaces/IAxelarGasService.sol).
 
-The process can be done through the Axelarscan UI, the Axelar SDK or via direct invocation of the Gas Receiver contract.
+## Add gas via the `AxelarGasService` contract
 
-When needed, our smart contract can receive gas to top up an existing General Message Passing (GMP) transaction in the following ways:
+To add gas to an existing GMP transaction through the `AxelarGasService` contract:
 
-### `addNativeGas`
+### `addNativeGas()`
 
-Pay additional gas to a transaction that needs it (referenced by its txHash) in native tokens on its EVM source chain.
+Call `addNativeGas()` to add additional gas payment using native currency after initiating a cross-chan call. This function can be called on the transaction’s source chain after calling the gateway to execute a remote contract.
 
-- In Solidity:
+In Solidity, `addNativeGas()` takes the following parameters:
+
+- `txHash` — The transaction hash of the cross-chain call
+- `logIndex` — The log index for the cross-chain call
+- `refundAddress`  — The address where refunds, if any, should be sent
 
 ```solidity
 function addNativeGas(
@@ -20,14 +24,20 @@ function addNativeGas(
 ) external payable override;
 ```
 
-- In JavaScript or TypeScript, the SDK abstracts a method that can be invoked directly in a web application.
-  See SDK docs for [Increase Native Gas Payment](/dev/axelarjs-sdk/tx-status-query-recovery#21-native-gas-payment).
+In JavaScript or TypeScript, the SDK [abstracts a method that can be invoked directly in a web application](https://docs.axelar.dev/dev/axelarjs-sdk/tx-status-query-recovery#21-native-gas-payment).
+
+
+### `addGas()`
 
-### `addGas`
+Call `addGas()` to add additional gas payment after initiating a cross-chain call. This function can be called on the transaction’s source chain after calling the gateway to execute a remote contract.
 
-Pay additional gas to a transaction that needs it (referenced by its txHash) in any of Axelar's supported tokens on its EVM source chain.
+In Solidity, `addGas()` takes the following parameters:
 
-- In Solidity:
+- `txHash` — The transaction hash of the cross-chain call
+- `logIndex` — The log index for the cross-chain call
+- `gasToken` — The ERC-20 token address used to add gas
+- `gasFeeAmount` — The amount of tokens to add as gas
+- `refundAddress` — The address where refunds, if any, should be sent
 
 ```solidity
 function addGas(
@@ -39,6 +49,7 @@ function addGas(
 ) external override;
 ```
 
-- In JavaScript or TypeScript: [Increase ERC-20 Gas Payment](/dev/axelarjs-sdk/tx-status-query-recovery#22-erc-20-gas-payment).
+In JavaScript or TypeScript, the SDK [abstracts a method that can be invoked directly in a web application](https://docs.axelar.dev/dev/axelarjs-sdk/tx-status-query-recovery#22-erc-20-gas-payment). It is similar to a native gas payment, except with Axelar-supported ERC-20 tokens instead of native tokens. Make sure that the ERC-20 tokens you use are supported by Axelar.
 
-\*\*\* **Can only be paid in tokens that Axelar supports. See the list of supported assets for the chains we support: [Mainnet](/dev/reference/mainnet-contract-addresses) | [Testnet](/dev/reference/testnet-contract-addresses).** \*\*\* 
+- [Supported Mainnet chains](https://docs.axelar.dev/resources/mainnet)
+- [Supported Testnet chains](https://docs.axelar.dev/resources/testnet)

src/pages/dev/general-message-passing/gas-services/intro.mdx

@@ -1,17 +1,39 @@
-# Axelar gas & executor services
+# Introduction to the `AxelarGasService` contract
 
-## Executor Service 
+[`AxelarGasService`](https://github.com/axelarnetwork/axelar-gmp-sdk-solidity/blob/main/contracts/interfaces/IAxelarGasService.sol) is a smart contract that manages gas payments and refunds for cross-chain communication on the Axelar network. It allows an application to prepay gas fees on the source chain and increase the gas paid for a transaction. If too much gas is paid for a transaction, it can calculate and refund the sender.
 
-Axelar network provides an optional relayer service, called the Executor Service, which observes the gas-paid amount to the Gas Service contract (below). Then, it automatically uses those amounts to relay the approved message to the application’s destination contract.
+## Prepay gas on the source chain
 
-Users are required to pay gas to the Gas Receiver contract to activate the Executor Service. After an attempt to relay the message to the destination contract, Axelar gas services calculate the remaining gas amount and [refunds](./refund) it to the payer account. The execution result can be monitored on the [Axelarscan](https://axelarscan.io) UI or requested through the AxelarJS SDK. Please see the section titled [Monitoring state of GMP transactions](/dev/general-message-passing/monitoring) for more information.
+The Axelar network routes every [General Message Passing (GMP)](https://docs.axelar.dev/dev/general-message-passing/overview) transaction to its destination chain. The final step of this process involves the application paying the gas fees for the transaction. This can be done in one of two ways:
 
-## Gas Receiver
-For any General Message Passing (GMP) transaction, the Axelar network routes transactions to their destination chains. Execution is the final step of the pipeline to the specified destination contract address on the destination chain. It is invoked in one of two ways.
+1. **On the destination chain:** Manually paid on the destination chain
+1. **On the source chain:** Prepaid to the `AxelarGasService` contract on the source chain, thus allowing it to execute the final transaction on the destination chain
 
-1. Manually paid by the user/application on the destination chain.
-2. Executed automatically by Axelar if the user/application prepays gas to the Gas Receiver contract on the source chain.
+It's possible for gas fees to be manually paid on the destination chain, but developers typically rely on Axelar's Gas Service, which provides the ability to prepay the full estimated cost for any GMP transaction with a single payment on the source chain. This service manages the full pipeline: once gas is paid to the Gas Receiver contract, the Gas Service uses the payment to submit the transaction to the Axelar Network and to execute the final GMP call on the destination chain.
 
-Gas Receiver is a smart contract deployed on every EVM that is provided by Axelar. Our gas services provide users/applications the ability to prepay the full estimated cost for any GMP transaction (from source to destination chains) with the convenience of a single payment on the source chain, relying on Axelar's relay services to manage the full pipeline. Once gas is paid to Gas Receiver for a GMP transaction, Axelar's relayer services pick up the payment and automatically execute the final General Message Passing call.
+## Increase gas
 
-Developers can use Axelar gas services by prepaying upfront the relayer gas fee on the source chain, thereby covering the cost of gas to execute the final transaction on the destination chain.
+If the amount of prepaid gas is not enough — for example, if the destination chain is congested with transfers, causing a gas fee increase for each transaction — the application can add additional gas to pay for the transaction.
+
+## Gas refunds
+
+After the message has been relayed to the destination contract, Axelar will calculate and refund any extra gas fees to the payer account on the source chain. This execution can be [monitored on Axelarscan or requested through the AxelarJS SDK](https://docs.axelar.dev/dev/general-message-passing/monitoring).
+
+## Two-way calls
+
+The Gas Service supports relaying two-way calls, when a message is sent from the source chain and another message is returned from the destination chain.
+
+- **Outbound call:** a GMP call from chain A to chain B
+- **Return call:** a GMP call returned from chain B to chain A
+
+Once the outbound call is executed on chain B and sends another call to return a message to chain A, Axelar will automatically pay the gas for the return transaction.
+
+If the remaining gas amount is insufficient for the return trip, the [**Insufficient Fee** tag](https://docs.axelar.dev/dev/general-message-passing/monitoring) will show up on Axelarscan. The return call will not be relayed until more gas is added via [Axelarscan](https://docs.axelar.dev/dev/general-message-passing/recovery#increase-gas-payment-to-the-gas-receiver-on-the-source-chain) or the [AxelarJS SDK](https://docs.axelar.dev/dev/axelarjs-sdk/tx-status-query-recovery#2-increase-gas-payment).
+
+### Sending messages from a single transaction to multiple destination chains 
+
+The Gas Service also supports relaying multiple message calls from a single transaction. To do this, the application must pay the `GasReceiver` contract a single gas fee covering all messages. The following message call is an example transaction where this occurs.
+
+- [Transaction on source chain](https://moonbase.moonscan.io/tx/0x25f0bdcdec0da17e1039161342603d3d537cb6ddc6637d1b22dbdf1ebf9706ed)
+- [Message #1 information](https://testnet.axelarscan.io/gmp/0x25f0bdcdec0da17e1039161342603d3d537cb6ddc6637d1b22dbdf1ebf9706ed:1)
+- [Message #2 information](https://testnet.axelarscan.io/gmp/0x25f0bdcdec0da17e1039161342603d3d537cb6ddc6637d1b22dbdf1ebf9706ed:3)