You dont really need it bro, you will get much better results tracking your calories and hitting your macro targets. All reta will do is reduce hunger and make it hard to eat lots at once. So unless you got fat because you cant control yourself, its unnecessary.
Cutting from a lean bulk is very easy, and if you just rely on reta and forget about tracking, you probably wont hit your protein and actually loose more muscle on the cut.
If your skinny you should try to eat as much clean whole foods, in about a 300-400 calorie surplus, i wouldn't really take it over 18% bf but hard to say for you idk your current bf.
hgh should not effect natural production, but any amount of test will. If you decide to come off, assuming you do want to or why else this would matter. You will need to undergo pct, and even then there is no guarantee it will work. There is a high chance you will be on test for life even to maintain normal levels. I wouldn't recommend gear. HGH will also not do that much for muscle growth peptides are all kinda cope compared to test, just dont waste ur money or sacrifice your health, the peptide grey market is very sketchy, massive risk for such little gain.