first thing to do is find presets you like, and them modify them. Makes life a whole lot easier.
secondly, one of the biggest tone-shaping elements of the G2 is the amp simulator. Go in and play around with it. You may well end up just turning it off for most sounds. I leave it on, cos I love it, but it's quite a pronounced change...
thirdly, make sure that any reverbs you use have a serious bass roll-off on them - reverb on the lower notes of a bass guitar can get really muddy, so unless you're looking for a really dark, muddy sound, pick a reverb with a cross-over and roll off the lows..
lastly, read the manual! There's SO much going on in the G2 (the section about controllers - external and internal ones like LFOs and the randomiser etc. is mind-boggling) I'm still learning more about it and I've had mine for 14 years!