Don't forget to put aside ~$4000-5000 for your glandular excision down the road to correct your gyno.
Let my outline this for you.
Best case scenario. You gain a bit more penis size through an enhanced recover process. This is much more likely with hgh than Test. Will your penis grow like it did at the onset of puberty, no. It won't. Your penis is fully grown and you aren't going to change that since you've already completed that stage of puberty. Testosterone is only a catylist of virilization in males, it does not directly act on the penis tissues. If it did, Pro BBers would be too hung for their posing trunks. I've been around my fair share of BBers and users, needless to say that isn't the case.
Worst case scenario. You spend quite a bit of money. It affect your penis at all, but you do gain some more muscle. You get caught and royally screw up your life. Or you get gyno and not only is your penis unchanged but you end up getting "manboobs". And trust me, its not the most fun of procedures to go through.
This is your choice and your body. But, using and form of artificial hormone before the age of 25 without medical supervision is very risky. Mainly because puberty is fully finished in most, something you admit to.
You are young, rather immature, and impatient. Do what you want, but please do some more research and make a truly informed decision.