All of this thread is assuming that most people go to gyms for the actual exercise rather than the socialising and posturising!
My pet hate is people who boast about having a "personal trainer", when generally that means they're too lazy to be able to workout on their own, or want to look a big shot.
Another infuriating one, is when someone has asked you as a runner, what are some good stretches to do, and someone who doesn't run, and is grossly unfit, butts in and starts answering for you.
One guy at my work does both of the above 