Of course you can use it without GH. Saying otherwise is like telling someone they can't use test with tren, or some ridiculous shit like that. Regardless of whether we use GH or not, insulin will still provide its benefits.
Doesn't the body already produce insulin every day, regardless of whether we exo GH or not? Yes--because it can and will continue benefiting from it. GH doesn't change this. Insulin is not dependent on GH in order to elicit its effects.
Certainly, using both will produce better muscle gains and make it easier to remain lean, but so will adding GH to testosterone.