First of all, I'm so proud of you for reaching out to get treatment. That is truly the hardest part! As a fellow anxiety sufferer, I can tell that it DOES get better. As someone who also had WLS I will tell you, from my experience, any amount of weight my meds may have contributed to is NOTHING compared to feeling healthy and strong and not carrying the burden of chronic anxiety. I was on treatment prior to WLS so its hard to say what impact, if any, it had. But even if I were starting today, knowing it might impact me weight, I would absolutely do it in a heartbeat. YOU ARE WORTHY OF GOOD MENTAL HEALTH.
That being said, Zoloft may contribute to a ~1% weight gain over the course of the first year of treatment, which is pretty negligible. And its helpful to remember that the med itself doesn't cause the weight gain, the changes in habits it can lead to are what cause the gain. So if you stick to your good habits (hit your protein, drink your water, move your body) you will be just fine.
We are rooting for you!