On surface level 1 you enter gate, and it leads you to level n-1 (to subterra level 1).
On subterra level 1 you again press space and gate leads you to level n-2 (to subterra level 2).
On subterra level 2 you press space and get back in subterra level 1, then to surface.
What if subterra level 2 has only 1 gate, and subterra level 1 has 2 gates (map editor was not able to do good map)? You can make reverse check. Is this level 2 gate is closer to first gate on level 1 than to second gate, it will only work with first gate.
Another way is to have subterra gates like monoliths - add several types of them and use only subgates of same type.