Yes you can, but you's better be running it on a fairly hefty system, Guitar Rig slows my 17" PowerBook to a crawl, I have to mute most of the tracks to set the tone I want, then export the audio out to iTunes and re-import back to GB as an audio track just to get it to run well.
I have to say, I rarely use it in GB, but I use it all the time in ProTools.