Yes, what Sticky said for 'after' the command is performed would, logically, be even better.
However, that stilll doesn't account for non ULX commands 'kick, kickid, ban, banid, etc'.
Those are server functions, and I'm not sure Garry ever made lua hooks for them.
I poked around the Gmod wiki. Seems Gamemode.PlayerDisconnected itself only kicks off when a 'normal' disconnect occurs (quit, etc)