DTGTM works in PvP
DTGTM does not work in the way many people expect it to.
DTGTM only works with physical damage taken.
DTGTM does not reduce the damage you take, it merely restores your mana by a percentage of the physical damage you take.
Quote (Arreat Summit)
Damage Taken Goes To Mana
This can generate extra Mana as a result of being attacked. It does not reduce the amount of damage to Life but instead just adds to Mana.
Quote (Diablowiki)
the real bummer of combing DTM and ES is that DTM's mana boost is applied to your mana BEFORE the mana loss from ES is calculated. In other words, your mana can still be reduced to zero by a hit. DTM won't ever save you from having NO mana.
I hope this helps