Is this possible?
 

To make multiple gmaps all containing the same levels?

For example, if I had a 4x4 gmap and all of the levels had entrances and exits at the same co-ordinates so that they could be arrange in any way and still make a useable layout could I then make multiple gmaps with the levels in different orders?
Would players in the same level be able to see eachother even though they might be on a different gmap?

And is there a player.gmap equivalent of player.level that could be used as a condition to hide and show different npcs based on which gmap a player is in?

It shouldn't, all levels should (and be required) to have a unique name. If you really need to repeat the same levels, just write a generator to do it for you or copy and paste it.

player.gmap.name - returns the gmap (world.gmap) name the player is in.

I imagine so, you have to manually make the .gmap file. Since the players are all in the same level, I think they would see each other. I believe the level takes priority over gmap. Try it and let us know. I can just imagine all the players staying the the top left most level (gmap wise) regardless.

This is what I thought as well.