Same company makes both but the GMC is considered and priced as more of a high end version of the Chevrolet pickup and SUV line thus there are far more Chevrolet models on the road than GMC models.
Well, since GMC only makes trucks and SUVs, while Chevy has a full line of vehicles, Chevy is more widespread. But they are still both made by GM, so there really isn't a difference between GMCs and Chevy's trucks and SUVs
The USA is the biggest purchasers of trucks and SUVs....in the entire world. Although Ford takes the crown for actual yearly truck sales- GMC Denali and Yukon SUVs outsell Ford/Lincoln SUVs, by a huge margin. One of the reason for this is the State Department exclusively uses black GMC SUVs for everything from shuttling our(current) stupid President around, to Homeland Security/Big-Brother uses.