How are GPAs from different universities evaluated for admissions to MS programs in the USA?
We have the same phenomenon in America, of course. All other things being equal, a 3.5 GPA from Princeton is much more impressive than a 4.0 from some random state university. The whole idea of GRE scores is to provide a metric that isn't confounded by the difficulty of different programs at different colleges in these ways. An additional difficulty you face is that  your American evaluators won't know how good or bad your university is. This makes your GRE score doubly important.