Kill 'em all and let God sort 'em out. Seriously, just let it go. Some people praise others by being sarcastic and cutting them down. More so, they try to make themselves feel worth a shit by doing things like this. Stay focused on what you're in the gym to do, don't make eye contact with them and just act like they don't exist, and get the hell out of the gym when you're done. Who can take 2 or 3 guys seriously that pick on a minor? My god, sounds like they need therapy or some gay porn and a crate of anal-ease.
If they keep up, you could always go up to them and start talking. Joke with them and ask when they are going to compete. I wouldn't start any shit ... they sound like the "Deliverance", redneck types. They'll just tie you up like a helpless barn-yard animal and do naughty things to you.
PS. Congrats on winning the cancer battle. My stepmom has liver cancer which she got after having breast cancer. It's not a good situation right now for her and I pray for her and my dad every day. Your battle with the dipshits should be a walk in the park compared to what you've already been through.