To address the level of players for the dungeon, try several GS's.
Players 1-20 in GS1
Players 20-40 in GS2
etc.
You get the idea. This also gives lvl 75, 76 players an area to PvP without the interference of lvl 200 players ganking them.
Perhaps an overlap would be necessary. Players that are right at the top of on GS, but bottom of the next.
Entrance could be gained via a whompa in ICC. Since ICC is the connecting area for all Whompa systems.
Just a thought anyway.