Sorry I do not have more time to investigate with developing the new IDE full time. I just want to post to let you know I think I might know the answer to your problem. After configuring an external code editor you may need to reopen any scripts/code windows or completely restart. When you do, you should then see a pencil icon on the toolbar of the code editor. That pencil icon will open the script/code in the external editor. We could discuss improving on this in LGM, but iirc GM was always the same way, and I am no longer supporting LGM with the new IDE on the way.
You should check the following Wiki guide instead of the other one:https://enigma-dev.org/docs/Wiki/Overriding_settings
The one Hugar is referring to is just how to make Notepad++ have GML syntax highlighting which is independent of using it with ENIGMA or GM.