I'm a HUGE Sonny and Will fan, and I don't care if it's never revealed what Gabi "did". Gabi's a criminal, Will's a criminal, Nick is a criminal, Sami's a criminal, Lucas is a criminal, and Sonny's a criminal. That baby is surrounded by criminals so it doesn't matter if one goes down for what they did because you still have a clan of criminals taking care of that child.
I don't like Gabi because even though she doesn't know all of the details of what happened between Will and Nick, she knows enough to where she should understand why Will doesn't want Arianna around Nick. Will didn't forbid her not to see Nick, he just doesn't want him around Arianna, which given the fact that he blackmailed Will into signing over his rights, is completely understandable. I don't give a rats ass who Gabi sleeps with, but I do care if her romps with Nick jeopardizes Will's relationship with his daughter.