So, I'm ready to leave the darned place (gotta go back upstairs and find Vetrian first) and I can't get this gate open. According to the walkthrough, it looks as if the only way to pass this gate is to edit my position on the map, since there's no noclip cheat. I think I'm supposed to decrease the Y value (moving me North?), but i'm not sure. Has anyone a helpful tip?
You accidentally closed the gate when you passed through - it can only be opened from the other side. Unfortunately, you won't be able to edit your position within the dungeon. The best way out is to edit your position to Arindale (or, if there is a preset position at Ulm), and then go back into the dungeon to save Vetrian.
Click the map tab, and look at the bottom right frame, labeled "Location". Click the first drop-down menu, and select "00 - World Map". Then, in the second drop-down menu, select "Town-Arindale".
Make sure you remember to correct the experience/advance numbers under the Stats tab before you save.