purchase

export default {
  name: 'MyComponent',

  methods: {
    track () {
      this.$gtag.purchase({
        "transaction_id": "24.031608523954162",
        "affiliation": "Google online store",
        "value": 23.07
      })
    }
  }
}

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