I was in a simular situation at my old gym. There was this one dweeb who I had never even said a word too who spent all his time talking smack to others about how I must be on the most drugs in the gym, etc, etc. What was funny is that I found out that he had been using lots of juice before I even started, and he still looked like crap, in fact at the time of his most vocal accusations I had been clean 4 months and was still twice as big as him. Maybe that is why the guy who works at your gym is so jealous. He uses juice but because he does not train/eat right and has shitty genetics he is still a skinny shit, albiet a skinny shit with more acne, so he takes out his frustration by bashing you and claiming he is natural.