As long as you can export from JU to GB you should be OK with the iPAd1... I just received an iPad1 from my employer and loaded it with all these music apps the guys are using here. While it works fine with most of them (any that don't require iOS6) the iPad1 is NOT up to multi-tasking as is often done (ie: instances of JamUp, AudioBus, and MTDaw running together). I've given up on using the iPad1 one for multi-tasking with music in the manner typically discussed here.
I haven't tried running a backtrack and jamming to it (within JamUp) with the iPad1 yet...but if you keep it simple and all in one program you may be alright. If GB is necessary in the process, as you suggest, I'd use JU for the bassline and get it to GB to do what you need to do next.