I pulled up some photos from the web of both a GMC and a Chevy Suburban from 1989 and they look to be the same size and shape, but the GMC design is different and looks like it may stop short of the headlight housing. It probably comes as one big piece, but you may want to look into that.
I also found some aftermarket companies that appear to suggest that the GMC and Chevy grills can be interchanged, but they want to see you everything, including the headlight housings.
There may be differences between the 1500 and 2500 Suburbans too.