Yes I agree with Eeeb that it's a genuine watch movement.
Actually the number of vintage forgeries is pretty low, and it'll more likely be a Rolex or a very high end watch that is counterfeited. GP is pretty nice but not popular enough to fake.
What is more of a vintage problem is some joker putting together a Frankenwatch - mixing movement, case and dial to make what looks to be the real deal. This is harder to detect in many situations. But I don't think your watch is one of those either.
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