What do Phoebe, Yvonne, and Ron actually see in Gary? He's selfish, lets them down all the time, lies constantly, and often disappears for weeks at a time. Granted, he keeps Phoebe supplied with stockings and bananas and can take her shopping in Harrods, but once Yvonne becomes a millionaire in Series 5, she doesn't really need him for anything. It's not like they were very affectionate to begin with. He also never does anything nice for Ron, who is well aware that Gary just uses him to print 1940s fivers for him.
Gary is charming enough in a superficial sort of way that you can see how Phoebe and Yvonne could fall for him at first blush. And he is clearly trying a lot harder with the former than he ever did with the latter. Ron doesn't seem to have many other friends, maybe he is lonely enough that even Gary is preferable to loneliness.
For Ron, as suggested by the above, Gary is his only friend. When Ron's having trouble with his marriage, Gary is the only person he can talk to. It's telling that, though Gary isn't the most dependable person, when Ron is arrested, Gary is the only one Ron thinks to ask for help. He really has no one else in his life for most of the series. Also, while you might say that Gary never does anything nice for him, Gary was the one who took him in when Stella chucked him out and let him live mostly rent free in his Mayfair apartment. Though their relationship is obviously biased in Gary's favour, Ron does get more out of it than he does from anyone else. Also, Ron seems to find their friendship quite exciting and he sees the advantages having a mate who can time travel has to offer (e.g. using Gary to invest in stocks or to get parts for his brother's jeep) to the point where, when it looked like Gary was going to be trapped in the past in "Just in Time", Ron immediately tried to become friends with another time traveller.
For Phoebe, there's no mystery to why she likes him. Gary is exciting, he romances her, he buys her things, she doesn't know he's lying to her, he doesn’t let her down that often and most of the time she thinks he had a good reason if he did (only really getting upset when she doesn't realise why he's letting her down, like when he refuses to pursue a music career). She’s also a girl from the east end who’s never had much, whose only real relationship was with a man she didn’t love, who’s being pursued by (what she believes to be) a rich, interesting, brave, talented musician and spy who loves her. Of course, she has no idea that when he disappears for weeks at a time he's doing it to be with another woman and thinks he's doing it for his country. She also does get to see the best side of Gary (e.g. when Gary steps up and helps teach the neighbourhood kids).
For Yvonne, they were married before the show started and their feelings for each other would have developed long before Gary became a time travelling adulterer (which is where the selfish behaviour you mention comes from). Like Phoebe, Yvonne doesn't know he's lying (so that's not really relevant to her opinion), merely that he's not dependable (which she does get angry about) and she thinks he's busy with his work (like she herself is). Throughout most of the show, they were planning on starting a family which was a big thing she wanted which kept them together. Late in the show, sure she doesn't need him but after years of marriage relationships can have inertia which keep them going unless there's a big reason for a breakup. This is especially true when people have been through the hard times together. Besides, by the later series Yvonne is sufficiently independent that someone who disappears for days on end probably suits her more than someone who will be around all the time and will want her to do the same. They also seem to be attracted to one another and to have a healthy sex life.
