I'm actually having trouble thinking of a Gildan level shirt that is made in the USA; I can think of a few companies with US manufacture, but they tend to make way more premium (and therefore more expensive) products than that. There have been a few threads in the past on the forums about union made shirts; I'm guessing that you'd have better luck finding a more ordinary, loose cut shirt (as opposed to soft fashion cut style) amongst those brands.
Bayside, maybe?
Might find some leads here:
union related topics at T-Shirt Forums