As explained by Alan Sepinwall:
After he returns to his own home (as opposed to the safe house on the other end of the tunnel), opens the shutters to let daylight back in, and instructs Mike to resume construction on the Super Lab, Gustavo Fring allows himself an indulgence, but only briefly. He heads to a local restaurant to enjoy both the wine and the chance to gently flirt with its charming sommelier, David.
“Hermanos,” the Breaking Bad episode featuring the flashback where Hector murders Gus’ partner Max, implied that Gus and Max were lovers — or, at least, that Hector believed them to be. (And also that Hector’s gay panic was where his resentment of and suspicion toward the Chicken Man began.) BB never explicitly said this, and Giancarlo Esposito has talked in the past about how he appreciates the ambiguity about how many layers existed within that partnership. Here, though, there is no mistaking Gus’ feelings regarding David. It is a remarkable scene to witness, because Esposito gets to play a Gus unlike any we have seen before on either series. To date, there have been three flavors of Gus: 1) the exceedingly polite local business owner who would love to offer you some of his signature spice curls; 2) the stoic, tight-lipped cartel executive; and 3) the avenging angel who will never get past that moment when Hector Salamanca put a bullet in Max’s head. This man at the wine bar is very clearly the same as those, yet he is unlike any of them. He is… enjoying himself. He is reserved in speech, not because he is afraid to betray any weakness to potential enemies, but because he just wants to hear this beautiful man talk about wine. He is still precise in all things, as we see in the way he prepares to drink the expensive wine David has poured for him. Yet he is almost unnervingly relaxed, relatively speaking, because of the pleasure he gets from being around the boy he likes.
WRT the end of that scene:
The problem, as Inigo Montoya might tell you, is that when you devote your life to the revenge business, you have precious little room for anything else. When David steps away for a moment, Gus’ contented expression hardens into his more recognizable paranoid mask. Gus is a man in the midst of an elaborate, multiyear plot to destroy his enemies while making himself fabulously wealthy. Going any further than very occasional chatting with David (who implies he hasn’t seen Gus in quite some time) would be dangerous for both of them. It is one thing for Hector to imply that Gus is gay, and another for anyone in the cartel to potentially find proof of it. And leaving institutional homophobia out of the equation, it is simply unfair of Gus to take on anyone as a romantic partner at a time such as this, because the possibility exists that they could wind up with a fate similar to Max’s. In the aftermath of vanquishing Lalo, Gus allows himself the smallest of celebrations, but that is all it can be. He leaves a generous tip on the bar, and exits before David can return to tempt him further.