Are you trying to have different levels on a gmap with different tilesets? That cannot be done (well it can, but it won't be the way you want it to be).
But if you want to same tiles for every gmap level (entirely possible), hopefully all of your gmap levels have the same prefix and you can just add to some kind of system NPC:
PHP Code:
addtiledef2("tilesetimg", "levelprefix", x, y, width, height);