Default value for slice/map is nil. So slice/map field will not be serialized if it is not initialized.

I mean from serialization point of view these two structs are the same:

struct {
  GroupKey *[]string `json:"group_key,omitempty"`
}

struct {
  GroupKey []string `json:"group_key,omitempty"`
}

The later version is more convenient to work with..

https://play.golang.org/p/0c0nVaoOPNK

Also the same applies to string type with omitempty json tag.

Pointer to interface *interface{} struct fields are also useless.