Short for Oh my Gackt; a play on OMG. It implies that you see Gackt as a God.
Note: Most people who use OMGackt use it to make fun of Gackt/Gackt fangirls.
Some girl said, "OMGackt!", but was actually serious about it, so Mitchi and I had to rip off her arms and use them to strangle her.