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There's a "3GS wheel thread" that has full of 100+ pages of offsets and fitment questions given to boost knowledge of various set ups and taste, but if you're looking for a quick answer given by your description of camber and fender mods... Then YES your 19x9/10 with +20 F/R can fit.

 

Yes I know... I do need a quick answer. I'm trying to get the wheels tomorrow. Thank you.

Do you think I would need any spacers to clear bbk? Will that setup requires any stretch?

 

The common offsets that fit and allowed other 3G GS owners to lower their cars was 35mm offset in the front and 42mm in the rear. Those specs were for 20x8.5 and 20x10 rims. 20mm offset will stick out past the fenders.
The rear spacing on the wheels will determine if you need to use spacers, not the offset of the rims. Before you buy them. Have the dealer test fit a set to see if it gives you the desired look.

Big Boyz with Bad Toyz