diff --git a/packages/gatsby-plugin-google-tagmanager/src/__tests__/gatsby-ssr.js b/packages/gatsby-plugin-google-tagmanager/src/__tests__/gatsby-ssr.js
index 1c451309fe16c..92e0fe323e051 100644
--- a/packages/gatsby-plugin-google-tagmanager/src/__tests__/gatsby-ssr.js
+++ b/packages/gatsby-plugin-google-tagmanager/src/__tests__/gatsby-ssr.js
@@ -30,6 +30,7 @@ describe(`gatsby-plugin-google-tagmanager`, () => {
         setPreBodyComponents: jest.fn(),
       }
       const pluginOptions = {
+        id: `123`,
         includeInDevelopment: true,
       }
 
@@ -51,7 +52,10 @@ describe(`gatsby-plugin-google-tagmanager`, () => {
       }
       const pluginOptions = {
         includeInDevelopment: true,
-        defaultDataLayer: { pageCategory: `home` },
+        defaultDataLayer: {
+          type: `object`,
+          value: { pageCategory: `home` },
+        },
       }
 
       onRenderBody(mocks, pluginOptions)
@@ -69,12 +73,17 @@ describe(`gatsby-plugin-google-tagmanager`, () => {
       }
       const pluginOptions = {
         includeInDevelopment: true,
-        defaultDataLayer: function() {
-          return { pageCategory: window.pageType }
+        defaultDataLayer: {
+          type: `function`,
+          value: function() {
+            return { pageCategory: window.pageType }
+          }.toString(),
         },
       }
 
-      const datalayerFuncAsString = oneLine`${pluginOptions.defaultDataLayer.toString()}`
+      const datalayerFuncAsString = oneLine`${
+        pluginOptions.defaultDataLayer.value
+      }`
 
       onRenderBody(mocks, pluginOptions)
       const [headConfig] = mocks.setHeadComponents.mock.calls[0][0]
@@ -96,7 +105,10 @@ describe(`gatsby-plugin-google-tagmanager`, () => {
       }
       let pluginOptions = {
         includeInDevelopment: true,
-        defaultDataLayer: 5,
+        defaultDataLayer: {
+          type: `number`,
+          value: 5,
+        },
       }
 
       expect(() => onRenderBody(mocks, pluginOptions)).toThrow()
@@ -104,7 +116,10 @@ describe(`gatsby-plugin-google-tagmanager`, () => {
       class Test {}
       pluginOptions = {
         includeInDevelopment: true,
-        defaultDataLayer: new Test(),
+        defaultDataLayer: {
+          type: `object`,
+          value: new Test(),
+        },
       }
 
       expect(() => onRenderBody(mocks, pluginOptions)).toThrow()
diff --git a/packages/gatsby-plugin-google-tagmanager/src/gatsby-node.js b/packages/gatsby-plugin-google-tagmanager/src/gatsby-node.js
new file mode 100644
index 0000000000000..d167c44bb02eb
--- /dev/null
+++ b/packages/gatsby-plugin-google-tagmanager/src/gatsby-node.js
@@ -0,0 +1,13 @@
+/** @type {import('gatsby').GatsbyNode["onPreInit"]} */
+exports.onPreInit = (args, options) => {
+  if (options.defaultDataLayer) {
+    options.defaultDataLayer = {
+      type: typeof options.defaultDataLayer,
+      value: options.defaultDataLayer,
+    }
+
+    if (options.defaultDataLayer.type === `function`) {
+      options.defaultDataLayer.value = options.defaultDataLayer.value.toString()
+    }
+  }
+}
diff --git a/packages/gatsby-plugin-google-tagmanager/src/gatsby-ssr.js b/packages/gatsby-plugin-google-tagmanager/src/gatsby-ssr.js
index dbcc2eeb55aa5..40f55eca1bb4f 100644
--- a/packages/gatsby-plugin-google-tagmanager/src/gatsby-ssr.js
+++ b/packages/gatsby-plugin-google-tagmanager/src/gatsby-ssr.js
@@ -14,16 +14,16 @@ const generateGTMIframe = ({ id, environmentParamStr }) =>
 const generateDefaultDataLayer = (dataLayer, reporter) => {
   let result = `window.dataLayer = window.dataLayer || [];`
 
-  if (typeof dataLayer === `function`) {
-    result += `window.dataLayer.push((${dataLayer})());`
+  if (dataLayer.type === `function`) {
+    result += `window.dataLayer.push((${dataLayer.value})());`
   } else {
-    if (typeof dataLayer !== `object` || dataLayer.constructor !== Object) {
+    if (dataLayer.type !== `object` || dataLayer.value.constructor !== Object) {
       reporter.panic(
         `Oops the plugin option "defaultDataLayer" should be a plain object. "${dataLayer}" is not valid.`
       )
     }
 
-    result += `window.dataLayer.push(${JSON.stringify(dataLayer)});`
+    result += `window.dataLayer.push(${JSON.stringify(dataLayer.value)});`
   }
 
   return stripIndent`${result}`