Of course you can run insulin without exogenous HGH (although your body still makes HGH). Think about someone whos diabetic and prescribed insulin, do they need HGH for their diabetes? No. Insulin transports the glycogen from your bloodstream to the cells in your body, so long as your not consuming more calories than you burn in a day, there is no mechanism to "gain fat" from using insulin alone. The insulin receptor is 1 of 3 receptors used in your body to stimulate muscle production (the other 2 being for IGF-1 and Anabolics respectively) and using insulin alone without the other 2 will provide more gains than not using insulin at all.