Any MIDI keyboard would work with GB as long as its set up right in Core MIDI.
jcroft - any 49-key keyboard is not going to be weighted. Weighting comes in the full 88- or 71-key sizes.
If I were looking to mess around, I would get a 61-key model that's USB, so there's no external MIDI interface needed. Many of the smaller ones (especially by M-Audio) are powered by the USB bus so there's no need for a power adapter, although they come with one.
Depending on your seriousness about it, it is worth the investment. A lot of programs accept the controls from the knobs to control effects and other parameters.
cpjakes