Context: some schoolgirls are talking about a girl that is dating a boy that killed someone when he was younger.
The only meaning of ガチ I could find was "serious, diligent", but I don't think it fits the context. Judging from what the girl says (攻められるプレイ) and from results in Google for ガチのM, I guess the M stands for masochism. Is this correct? If yes, could you explain the meaning of ガチ and how it can be used to form words? Thank you for your help!