First of all, Gillian didn't want Kirk. She wanted to stay with the whales. And Gillian didn't spill the beans about Gracie's pregnancy - Spock did. As for Kirk expecting Gillian to pay for the pizza, I've already made this point in another thread on another topic: Kirk is unfamiliar with cash. Not money as a means of economic exchange, but cash. We don't know how much he's spent up to that point, but we do know that Kirk has no idea whatsoever about the value of things. As he said to the crew: Don't splurge. The money they got from selling his glasses is all they've got. Plus, Gillian invited Kirk. Of course he would expect her, as the one who extended the invitation, to pay. The beer was nothing more than product placement.