It's a substitution code, but not all that simple. It's just in caps because it's in caps. It's grouped in 5s because that's kind of a tradition with coded messages. Actually there's another reason it's in caps. And the font it's in make the G kinda wrong for the use I'm putting it to. You can take that as a hint of some sort. I'm just trying it out here to see if it's too simple to try with some group of real cryptanalysts.