This is a very regional joke, and of course explaining it will make the joke not very funny, but I’ll explain it anyway.

I already explained in this comic how the G train is pure and simply the worst subway line in the entire system. A friend of mine recently put in application for a new apartment and it’s still on the G train. I said to him half-jokingly “Okay, your next apartment isn’t going to be on the G train.” I say this for myself and for him, because his friends (myself included) will be in a better mood when it comes time to visit him, and those occasions will usually be special occasions, because we want to avoid the G train at all costs.