Go into your Nightmare folder in your file explorer (so not in Nightmare). Then, go into your FE7 GBA modules and into the chapter data editor folder. You should see a bunch of text documents.
These are what make up the chapter data editor, and they (like all Nightmare modules) are easily editable for your needs.
When you see a .txt with a number at the top (13 in your case with objectsets.txt), that's the number of entries in this file. Change it to 14. When it's 13, Nightmare won't read the 14th entry, which is the Dragon's Gate.