I had very little problem with weight gain at the high doses of pred for GCA. However when I reduced to around 15 mg the weight started piling on, both surprising and discouraging. I have lost weight with carb restricted diets prior to GCA but now the carb restriction affects my mood in a depressed way. I hate the extra weight and bloating but I guess I have to save the dieting once I'm off Pred. I do eat a low sugar, gluten free diet and take several anti/inflammatory supplements to help the healing process.
I've just said on another thread - weight gain as you REDUCE the pred dose is very unusual! But there are now 3 people currently with the same problem. I wonder what it is.
Try substituting high protein for some of your carbs. I eat pumpkin seeds and eggs (no not on the same plate) for breakfast now, instead of cereal, no longer eat sandwiches or pasta on a regular basis but eat a lot of nuts as well as cheese. Meat eaters have even more options. Get your carbs and sugar from vegetables and fruit, not grains or sweet goodies.