You can get a 190 onto the 5.5" but, as Neil said, having the profile matching the front is important. Saying that, it would be difficult to get a 17" front that didn't match the same brand /model 17" rear tyre...
The main problem is the top-heavy nature of the GS and the height of the tyre profile making it harder to get the bike to switch from side to side on the rear. You just have to counter-steer like your life depends on it