did anyone wonder how the talk went between sean and his brother because he obviously told him about their relationship because then he comes to the diner to talk to zach about it
just be like bro im sleeping with your best friend
actually I think Gabe figured it out on his own without talking to Sean.
If you remember when Gabe bursts into the house Zach hides in the closet and without knowing it grabs one of either Gabe's or Sean's shirts as he rushed to escape without being seen by Gabe. In the next scene you see Zach working at the dinner. Gabe comes to say that they should go hang out now that he is back in town. Gabe then looks and Zach's shirt and makes the side comment "nice shirt" only then does Zach realize the shirt he's wearing isn't his, and Gabe knows that he's been at their house.