Taking my Orioles bias out of the loop, it should be Gil and it shouldn't be close.
Cowser had a nice season, and he's ending it off good, but Gil has 15 wins, a +126 ERA, and has been worth 3.5 rWAR to Cowser's 2.9 rWAR. Plus he's struck out a sexy 166 in 146 innings going into tonight.