Tremec is an independent transmission manufacturer. Vettes, Camaros, Mustangs, Vipers and a lot of other cars come with them as standard equipment. Tremec probably makes a half a dozen trannies that I know of, maybe more.
Over the years GM has used the same basic model Tremec manual trannies as Ford, but usually GM specifies different gear ratios than Ford. In addition you'll find the GMs have a different bell housing and possibly a different tail shaft.