You should contact your surgeon to be sure, or at the very least check with the pharmacist, but as I understand it you cannot take NSAIDs after bariatric surgery, right? Celebrex is not an NSAID, but a COX2 inhibitor. However, looking at drugs.com, the most common side effects listed for Celebrex seem to be related to the gastrointestinal system, so it's definitely worth talking to your surgeon.
Weight gain is not a common side effect for the medication. I took Celebrex for quite a while years ago and never experienced any at all. However, the information says that if you do have any weight gain, it is a severe complication and you need to notify your doctor immediately. http://www.drugs.com/sfx/celebrex-side-effects.html