You say DHT gel won't shut down natural testosterone production. I wish this were true, because if it was, I would be rubbing that shit all over my body (except my scalp). The truth is, any form of androgen, EVEN ENDOGENOUS TESTOSTERONE (from one's own testes) will send negative feedback signals to the hypothalamus. Everybody has a set blood level of androgens that their body fights to keep, and no matter where the testosterone comes from, it will tell the hypothalamus "alright, slow it down, we've got plenty" and before you know it, your testicles will temporarily shut down testosterone production until the testosterone is metabolized, blood levels fall, and the hypothalamus sends the signal (LH) to the testes to make more.
It is a myth that any androgen will not reduce endogenous output. Even HCG (which is an LH analogue used in males to increase ENDOGENOUS production of testosterone) will cause negative feedback. Each person has their own set blood level of androgens which is very difficult to overcome other than long-term injections of the stuff.
-Gold