diff --git a/packages/gatsby-plugin-google-tagmanager/package.json b/packages/gatsby-plugin-google-tagmanager/package.json
index d6ccdeb71ffef..a763748c5d7db 100644
--- a/packages/gatsby-plugin-google-tagmanager/package.json
+++ b/packages/gatsby-plugin-google-tagmanager/package.json
@@ -13,7 +13,8 @@
     "@babel/cli": "^7.11.6",
     "@babel/core": "^7.11.6",
     "babel-preset-gatsby-package": "^0.6.0-next.0",
-    "cross-env": "^7.0.2"
+    "cross-env": "^7.0.2",
+    "gatsby-plugin-utils": "^0.3.0-next.0"
   },
   "homepage": "https://github.com/gatsbyjs/gatsby/tree/master/packages/gatsby-plugin-google-tagmanager#readme",
   "keywords": [
diff --git a/packages/gatsby-plugin-google-tagmanager/src/__tests__/gatsby-node.js b/packages/gatsby-plugin-google-tagmanager/src/__tests__/gatsby-node.js
new file mode 100644
index 0000000000000..5315f095b1664
--- /dev/null
+++ b/packages/gatsby-plugin-google-tagmanager/src/__tests__/gatsby-node.js
@@ -0,0 +1,35 @@
+import { pluginOptionsSchema } from "../gatsby-node"
+import { testPluginOptionsSchema } from "gatsby-plugin-utils"
+
+describe(`pluginOptionsSchema`, () => {
+  it(`should validate valid options`, async () => {
+    const { isValid } = await testPluginOptionsSchema(pluginOptionsSchema, {
+      id: `YOUR_GOOGLE_TAGMANAGER_ID`,
+      includeInDevelopment: false,
+      defaultDataLayer: { platform: `gatsby` },
+      gtmAuth: `YOUR_GOOGLE_TAGMANAGER_ENVIRONMENT_AUTH_STRING`,
+      gtmPreview: `YOUR_GOOGLE_TAGMANAGER_ENVIRONMENT_PREVIEW_NAME`,
+      dataLayerName: `YOUR_DATA_LAYER_NAME`,
+      routeChangeEventName: `YOUR_ROUTE_CHANGE_EVENT_NAME`,
+    })
+
+    expect(isValid).toEqual(true)
+  })
+
+  it(`should support defaultDataLayer as a function`, async () => {
+    const { isValid } = await testPluginOptionsSchema(pluginOptionsSchema, {
+      defaultDataLayer: () => {
+        return {
+          originalLocation:
+            document.location.protocol +
+            `//` +
+            document.location.hostname +
+            document.location.pathname +
+            document.location.search,
+        }
+      },
+    })
+
+    expect(isValid).toEqual(true)
+  })
+})
diff --git a/packages/gatsby-plugin-google-tagmanager/src/gatsby-node.js b/packages/gatsby-plugin-google-tagmanager/src/gatsby-node.js
index daa6532e1b3f6..82b02708dec20 100644
--- a/packages/gatsby-plugin-google-tagmanager/src/gatsby-node.js
+++ b/packages/gatsby-plugin-google-tagmanager/src/gatsby-node.js
@@ -22,7 +22,8 @@ exports.pluginOptionsSchema = ({ Joi }) =>
       .description(
         `Include Google Tag Manager when running in development mode.`
       ),
-    defaultDataLayer: Joi.object()
+    defaultDataLayer: Joi.alternatives()
+      .try(Joi.object(), Joi.function())
       .default(null)
       .description(
         `Data layer to be set before Google Tag Manager is loaded. Should be an object or a function.`