[Admins]

For seeing players moving through the surrounding levels on the old maps you and the other players need to have UDP enabled. For completely classic servers it is the only available map type, but it is only clientside and doesn't display npcs or so.
The only possible problem with gmaps is that it can lag if you don't setup it correctly. You need to keep care that the npcs don't contain a lot of client-side script (which slows down the level entering) and don't do too many timeouts in clientside scripts (which would lag all the time because the scripts of all npcs in the 3x3 levels surrounding you are running). This is not really a gmap-problem, but it is making things worse if you already have slow/laggy scripts.
Advantage is that crossing a level border doesn't really lag anymore, npcs can move to other map parts too. That is making level editing simplier, and you can do things like trains, cars, boats and other ways of transportations. Also bullets (shoot command) are going more far and don't stop at the level border.