The current GS-1 is "Flat" (no slope correction). This is an older (unfortunately discontinued) 20-MOA GS-1 used by SFC Jason Saint John, USAMU Pistol Team NCOIC and multi-time winner of the Fort Benning International Sniper Competition.
The rail allows proper eye placement for the prone and prone/sling firing positions using medium height rings -- the scope can be swapped out with your other weapons.
They work great. Only problem I have run into with them is they are not quite 1913 spec. The slots are not quite wide enough to fit the cross bar on mil-spec rings. I found the Burris Zee rings worked well.