The suppressive effects are dose dependent. There have been a few studies that have looked at DHT gel or injection in men. Most of them, but not all show a suppression of T, E, FSH and LH.
Don't know why it might cause gyno. In boys it is used to treat it. The thought is that it suppresses endogenous T thus lowering E but maintains androgenicity and libido. Problem is, DHT has little effect on muscle and/or fat mass.
W6