"reference types" is a very specific concept from a specific category of languages: types which are always heap-allocated and sitting behind an invisible (and un-interactible) pointer.
But Go doesn't have that distinction, and has actual pointers you can use directly. A map is just a heap-allocated structure sitting behind a pointer.
If you create a type which is a pointer to a struct, sure you can say you've built a reference type if you want, but that doesn't actually say much to anyone, because that's not a distinction the language makes, unlike Java or C#.
But Go doesn't have that distinction, and has actual pointers you can use directly. A map is just a heap-allocated structure sitting behind a pointer.
If you create a type which is a pointer to a struct, sure you can say you've built a reference type if you want, but that doesn't actually say much to anyone, because that's not a distinction the language makes, unlike Java or C#.