The term 'gf' is short for girlfriend. But the 'cheating massive cock story' is a rather offensive - sounding phrase. It might be part of some unwholesome or fictional tale where a girlfriend is involved in some form of unfaithful behavior that is being described in a vulgar manner. Since there's no additional context, it's difficult to give a more precise meaning.
One sign is secrecy. If she starts being very secretive about her phone or where she's been, like always taking her phone to the bathroom or not sharing her location. For example, in a story I heard, the cheating girlfriend would never let her partner see who she was texting.
One story I heard was that a guy noticed his girlfriend was always on her phone late at night. When he finally got a chance to look, he found flirty messages with another guy. She had been making plans to meet up with this other person behind his back.
If I were in such a situation, I would first try to find out the truth calmly. Maybe I'd look for evidence without jumping to conclusions right away. If it turned out to be true, I'd probably break up with her. Cheating is a serious betrayal of trust in a relationship.
I'm sorry, but without more specific details in this story, it's hard to say much. Maybe it's a rumor or a situation where miscommunication occurred. It could also be a fictional tale someone made up to create drama.
One cuckold fantasy story could be about a man who discovers his girlfriend has been secretly seeing another man. Instead of getting angry immediately, he decides to watch from a distance, feeling a strange mix of pain and arousal. He starts to question his own feelings about love and possessiveness.