I am sorry to hear about your cat being diagnosed, but you will find many people who want to help. I am relatively new to FD, my cat being diagnosed in March, in remission in April and relapsed in August. Everything I have seen says most cats lose weight, but my cat, Missy gained, so while it is not typical, it DOES happen. She has been steadily losing (at a safe rate) since getting her to accept wet cat food. Once we got some insulin in to her and GOOD food, she stopped begging for food and continues to lose.
Good luck with your kitty, and while this is not fun, nor easy, at first, it IS manageable.