So the issue here seems to be that you can't use GPU scaling at 75Hz for some reason? Even with custom resolution the GPU scaling doesn't work at 75Hz? I'm not sure what to tell you. GPU scaling should work regardless of the refresh rate.
It can be tricky sometimes to get a game to run at the right refresh rate. Like Crysis and Crysis 2, for me they were always stuck at 24Hz. I fixed it somehow eventually.
Sorry but I can't really think of anything else to suggest.