config

export default {
  name: 'MyComponent',

  methods: {
    track () {
      this.$gtag.config({
        'cookie_prefix': 'MyCookie',
        'cookie_domain': 'blog.example.com',
        'cookie_expires': 28 * 24 * 60 * 60
      })
    }
  }
}

Remember, this is just an example. check the all API parameters in the official gtag documentation here