Back when, you might've been called a strongman. Or a physical culturalist, lol.
These days, yeah, I dunno. "Gymrat" just doesn't have that ring to it, right?
But if you're considering competing in powerlifting, well, you could say that you're a future powerlifter. Eh, no. Still doesn't sound right...
edit: "Powerbuilder" could work. Might confuse people though.