What makes Gabe do that, anyway. Especially considering what he has done?
I feel like there are only two possiblities.
1. He is scheming something and wants the groups to be at odds. (unlikely imo)
2. He has finally lost it. (most likely imo)
If he is scheming I can't see one thing he would gain. So It seems that returning to a civilized area has driven him back to extreme guilt. This means that perhaps he is making the selfish decision to turn on Rick's crew to redeem himself. Which is ludicrous and borderline insane considering what they have done for him.