If you are gaining weight while using insulin that is telling you that you are eating too many carbohydrates (sugar/starches)

Body Fat is the result of Insulin (which you are injecting now) and Excess Blood Sugar

The insulin is helping you use the blood sugar for energy - that's what it is supposed to do, but if you put too many carbs into your system (which leads you to inject more insulin to deal with the sugar level) the leftovers have to go somewhere when all your cells are "stuffed full"

And the only other option is "make more body fat"

Cut back on your carbs which will likely allow you to reduce your insulin dose a bit AND you will not only stop gaining weight, you will likely lose a bit of what you put on

Good answer