You asked for my European experience.
No gates at T-bars. No use, on the contrary: you have to line up as fast as possible, there´s little time between the T-bars coming. Two skiers are a "team" which can organize well. If not, the worst to happen is either just one or none going up.
Lifts: most new quads (Doppelmayr, Leitner, Poma) have them. It seems to me that they are part of the installation. They are a must when the lift has non-detachable seats and that moving carpet (or how you call it in English exactly).
In quads with detachable seats (they slow down before boarding them) the gates seem to be unnecessary for experienced skiers but they are probably of some help for novices. A quad at my home hill used to let them open permanently but there were even more confusions in the high season with lots of inexperienced skiers.
The gates seem to be a necessary organization tool in 6- or even 8-packs.
Haven´t seen them in gondolas - again, no use, counterproductive.
A lot of - probably most or even almost all (I never paid a special attention) older lifts don´t have them. They are not quads, though.
If you have a liftee standing at the right point and organizing people you probably don´t need the gate so much in a typical quad. If the line is completely self-organized, it seems to be better to have them.