you should be able to go to any dealer and get warranty work done, they get paid so they dont care where you bought it.
when the head gaskets blew on my 03 gmc 2500 hd it was towed to the closest dealer, a chevy, buick, caddy dealer.
most dealers around me are multi brand too. gmc dealerships tend to be stand alone set up, geared more to work trucks, with a few nice trucks tossed in the mix. my local gmc is also a isuzu dealer. waiting room is none existent, no loaner car or trucks either from this dealership.
i just had a recall done on my regal at a chevy dealer whos the states leading vette sales dealer. so they should take you anywhere.