package main

import (
	"fmt"
	"net/http"
	"io/ioutil"
)

func main() {

	url := "https://api.sandbox.{id}.gr4vy.app/payment-method-definitions"

	req, _ := http.NewRequest("GET", url, nil)

	req.Header.Add("Authorization", "Bearer <token>")

	res, _ := http.DefaultClient.Do(req)

	defer res.Body.Close()
	body, _ := ioutil.ReadAll(res.Body)

	fmt.Println(res)
	fmt.Println(string(body))

}

{
  "items": [
    {
      "id": "bank",
      "icon_url": "https://api.sandbox.example.gr4vy.app/assets/payment-method-definitions/bank.svg",
      "display_name": "Bank",
      "long_display_name": "Bank (BACS)",
      "method": "BACS"
    }
  ]
}