Alright, here's the breakdown:
GP32 - Gamepark
GP2X - Gamepark holdings
XGP - Gamepark
XGP mini - Gamepark
XGP Kids - Gamepark
The only GPH console is the GP2X. GPH was formed out of Gamepark because of differences in opinion on the direction the next console should take--with GPH favoring a more open-source, homebrew platform and Gamepark wanting a more commercial system (like the PSP and DS).
I believe that all of the XGP models will be open in the sense that software can be made for it by individuals, yes. The focus for the XGP will be high quality 3D and commercial games. XGP Kids seems to basically be a GP32. I'm confused about the XGP mini.. it runs GP32 bios (or a bios that looks identical to it), but it's not supposed to be compatible with GP32 software.
I'm going to be looking into this as I'm working on a major update for my Gamepark newbie site, but it would be nice if someone could confirm or correct what I said.