I heard back from G Systems.
The GP04 isn’t in stock. What is currently in stock: Gp03 (bleh), Gp02 (kinda cool but..meh) and Z Gundam.
Done. Z it is. My plan is to have this be a part of my collection that is not incorporated elsewhere. I originally wanted the RX-78-2, because I don’t have a model of the Gundam. I don’t have a model of the GP04 or Z either, and seems Z is pretty important to the Gundam universe, it will do the job quite nicely.
So now I just have to wait for the confirmation back from G Systems.
I’ll take full pictures when it arrives as well.