What's the difference between Chevrolet and GMC?

They look like the same vehicles but with different names. What's the advantage of doing this?

The only basic difference between GMC and Chevrolet are the grills and hub caps.

usually the only difference is grilles, and sometimes the interiors are a little different. GMC is usually slightly better equipped. Im guessing they sell more of the two, than they would if there was just one.

There isn't one, really. There are Chevrolet dealers and GMC dealers, and there have been for decades upon decades. GM just hasn't had the stones to tell the GMC dealers (the ones that are not also Chevy dealers) they're not getting any more cars. There are also contracts involved in that.

It might make sense if GMC were like an upscale version of Chevrolet, but they aren't.

It may have still made some perverse kind of sense when trucks were selling like hotcakes, but they aren't.