Insulin itself won't cause you to gain weight. It's simply a hormone, and everyone has it. Our bodies just don't produce it on its own, so we have to provide it - thus we are now the controller.
If the amount of carbohydrates you consume is more than that of what your body needs, insulin will cause you to gain weight because its allowing the excess glucose in your blood to be converted into energy. If you end up with more energy in your system than your system needs, it stores it as fat. Hence the gain in weight.
The hardest thing for us as diabetics is that if we let our diets get out of control, we end up giving ourselves too much insulin for our body's daily requirements. Allowing the excess storage of carbohydrates we consumed but didn't need. If we don't give enough, then we essentially starve our body of the energy it needs and it burns off fat - when we run out of fats, it eats into our muscle. You end up feeling terrible and in the same boat as when most of us were initially diagnosed - tired, irritable, hungry, weak, and in generally poor health.
A good way around this is to figure out what your system needs daily for carbohydrate intake. Plan your daily intake around that, combined with exercise, and you should have no issues losing anything extra that you have. You will have to work at it though.
Best of luck!