A gym nemesis is a person whose presence in the gym has a consistent adverse effect on one's own gym-going experience. The most common manifestation of such a person is someone who always seems to be on the equipment that you want to use but they could also just be an obnoxious, disrespectful, or otherwise unpleasant fellow gym-goer.
Mike: "Damn Joe, that neanderthal over there has been hogging the bench press for like an hour. And that's the third time this week! I think this guy might be single-handedly responsible for my under-developed pecs!"
Joe: "You know what you've got yourself there, don't you?"
Mike: "No. What?"
Joe: "You've got yourself a gym nemesis my friend."
Mike: "Fuck my life."