It doesn't matter what you drive....but if you're worried about what others think, just know that it'* those that don't drive what they build/sell that continue on all those stereotypes that I'm sure you've heard before from non GM ppl. Such as "see, GMs must suck, even their own employees don't drive them!!", and "how do they expect us to buy their cars, when they won't even buy their own??". Other employees really don't care though.
Although it is good to practice to support your own, you can drive whatever you want. You're not the only one who doesn't have a GM there. We used to joke about how technicians at the various dealerships here drive something other than what they work on. At Hyundai, most drive Fords, at GM, they drive Hondas, at Ford, those that don't own a Ford, own Mopars. My father worked for each of the big 3, and made it a point to drive what each offer.