Once other fish are established, the gams are quick snacks.
My little pond is going on 3 y/o now. GAMs have been in it for at least 2 years. With BG, GSF, WM and a few RES as their primary threat, they've managed to maintain a fairly resilient population so far. I've recently seen several with the distinctive black spawning spot on their abdomens recently, so I expect a new batch any time now.
I've actually stopped feeding pellets for about 6 weeks now to force the panfish to predate some of the YOY and minnows, and the GAMs are holding their own.