Yeah, the Giz is completely closed source and you can only run signed applications.
Firefly hacked the Giz to include his own signature, so anyone who applies his hack to the Giz can run his applications.
Anyone else who hacks the Giz can do the same.
People wanting to develope homebrew without hacking only have one option - to pass there homebrew onto Firefly and let him sign it. He's already stated a lot of rules though, like he will refuse to sign emulators for systems where the original hardware is still being sold (so there's no chance of GBA for example).
I can run my own app's on the Giz as I've hacked it myself, but no one else can run my apps, so it's kinda pointless to release them. I've no intention on releasing my hack either, as I don't fancy the support headaches which FF is already getting (A lot of people seem to be struggling to run his hack and homebrew)
GP2X is far friendlier. It's easy to develope for, easy to test, and the people using the homebrew know how to actually use it