I haven't had any weight gain at all but it could be a good sign that your body is absorbing fat, vitamins, minerals & all that good stuff that could have been impaired pre-treatment. My friend gained a lot when she got on Cimizia because she could finally eat whatever she wanted without getting stomach aches and she no longer just ate to get by. She ate because she loved food & those foods packed the calories so that could be an option too.
I've heard a lot of people say they've gained on a lot of the drugs to treat IBD so it could be possible the medication causes fluid retention or increased appetite but I've only heard of steroids doing that.