Not Lives

Not Lives (ノットライヴス) is a manga by Wataru Karasuma. It ran in the Dengeki Daioh manga magazine from June 2011 to October 2016, and is complete in ten volumes. Seven Seas has licensed it for English release.

Plot
Mikami Shigeru is a video game genius–he can play and conquer any game, and just as easily program his own. While planning his next project, Shigeru discovers a strange oddity among his stack of research materials–a game disc labelled NOT ALIVE. Absorbing into his body, the disc thrusts him into a strange parallel reality where he becomes the “Player” of a real life video game, one where he controls the mysterious and skilled Amamiya Kyouka as his new female avatar. With rules to learn, other players to combat, and an omniscient GM overseeing the process, will Shigeru’s gaming skills be enough to survive in this brave new digital world? (Official Synopsis)

Official
Official English site