gatsby-plugin-google-tagmanager

// In your gatsby-config.js
plugins: [
  {
    resolve: "gatsby-plugin-google-tagmanager",
    options: {
      id: "YOUR_GOOGLE_TAGMANAGER_ID",

      // Include GTM in development.
      //
      // Defaults to false meaning GTM will only be loaded in production.
      includeInDevelopment: false,

      // datalayer to be set before GTM is loaded
      // should be an object or a function that is executed in the browser
      //
      // Defaults to null
      defaultDataLayer: { platform: "gatsby" },

      // Specify optional GTM environment details.
      gtmAuth: "YOUR_GOOGLE_TAGMANAGER_ENVIRONMENT_AUTH_STRING",
      gtmPreview: "YOUR_GOOGLE_TAGMANAGER_ENVIRONMENT_PREVIEW_NAME",
      dataLayerName: "YOUR_DATA_LAYER_NAME",

      // Name of the event that is triggered
      // on every Gatsby route change.
      //
      // Defaults to gatsby-route-change
      routeChangeEventName: "YOUR_ROUTE_CHANGE_EVENT_NAME",
      // Defaults to false
      enableWebVitalsTracking: true,
      // Defaults to https://#
      selfHostedOrigin: "YOUR_SELF_HOSTED_ORIGIN",
      // Defaults to gtm.js
      selfHostedPath: "YOUR_SELF_HOSTED_PATH",
    },
  },
]

// In your gatsby-config.js
plugins: [
  {
    resolve: "gatsby-plugin-google-tagmanager",
    options: {
      // datalayer to be set before GTM is loaded
      // should be a stringified object or object
      //
      // Defaults to null
      defaultDataLayer: function () {
        return {
          pageType: window.pageType,
        }
      },
    },
  },
]