Going gluten free wouldn't cause weight loss on its own. Removing junky foods containing gluten is likely what caused it (white bread, cookies, cakes, etc.). Since gluten is beneficial for us, there's no reason to avoid it unless you have celiac disease.
But have you reached your ideal weight or are you overweight? I also ate a lot (and still do) since my ileostomy, and I don't feel bad about it at all - it's the first time in years that I can enjoy food, but I also choose healthy foods (next to no junk) and I track my calories.
Also, do you have access to a dietitian? I'd recommend speaking with one to make sure you're eating a healthy diet for your condition and if you're concerned about gaining too much weight, they'll help you get you on track.
Have you started exercising since your op or have plans to?
If fiber is a problem, go with the low-fiber options I listed above, or cook your food (cooked or canned fruits/vegetables aren't really problematic).
Even with Crohn's, fiber should only be limited IF you are in a flare - if you are in remission, then fiber will be beneficial as it allows fiber-eating bacteria to create a healthy gut flora, enhancing the immune system and also helping to control cholesterol.
What are you eating in a typical day? Do you keep a food diary or track your intake? Cronometer.com is a great place to track your intake online and see if you are overeating (calories).