Install the plugin using

npm i gridsome-plugin-gtag

Put your Google Analytics ID in you environment variables with a name similar to GOOGLE_ANALYTICS_ID.

Then add the following to you gridsom.config.js plugins array

{
    use: 'gridsome-plugin-gtag',
    options: {
        config: {
            id: process.env.GOOGLE_ANALYTICS_ID,
        },
    },
},