If you're doing all the wrong things, then it could be possible that your gym is making you fat! Many of us religiously work out at the gym with very little change. Read this and find out what you could be doing wrong and fix it:
Only cardio training: Cardio is great to lose weight, but to get muscle definition in your arms, lift your sagging muscles or get that six pack, you need to do some resistance training. And ladies, you will not get bulky muscles from lifting weights. In fact, you can't. Else, there wouldn't be puny guys in the gym trying to gain muscle.You miscalculate calories: Often people over calculate their workout calories, and under calculate those in their food. More calories in, and less calories out, means weight gain! So, don't eat those unhealthy samosas after your 30 minute walk.
Same ineffective workout routine: You've been doing the same routine, since the dawn of the century. So, you're body is used to it, and is so efficient at doing the workout, then it doesn't need to truly exert. You must keep changing your workout routine to make it effective.