example.graphql

type Foo {
   id: ID!
   name: String!
}
type Query {
  foo(id: ID!): Foo!
}  

pkg_model.go

package model 
import (
  	"io"
  	"fmt"
  	"strconv"
   	"github.com/vektah/gqlgen/graphql"
)

// if the type referenced in types.json is a function that returns a marshaller we can use it to encode and decode
// onto any existing go type.
func MarshalID(t uint64) graphql.Marshaler {
	return graphql.WriterFunc(func(w io.Writer) {
		io.WriteString(w, strconv.FormatUint(t, 10))
	})
}

// Unmarshal{Typename} is only required if the scalar appears as an input. The raw values have already been decoded
// from json into int/float64/bool/nil/map[string]interface/[]interface
func UnmarshalID(v interface{}) (uint64, error) {
	switch v.(type) {
	case string:
		return strconv.ParseUint(v.(string), 10, 64)
	case int:
		return uint64(v.(int)), nil
	case float64:
		return uint64(v.(float64)), nil
	}

	return 0, fmt.Errorf("unable to unmarshal: %#v %T", v, v)
}

types.json

{
  "ID": "pkg/model.ID"
}