I decided to contact GOG support and asked them, if they could add links to scenario editors for GNB1, GNB3, GNB4 and GNB5 (GNB2 was released without an editor).
Required *.exe files are already present.

Just FYi, so to speak...
... and feel free to do the same, they might actually add them in the end!

(yes, I know how to access them, but their setup in non-trivial, especially for the later games, as they require CD and it would be nice to have them accessible right away anyway)

Did some tests and found, that while GNB3/4/5 editors can be accesed easily, GNB1 has issues.

Unlike later games, which had just two executables (game and editor and both can be easily handled, as GNB3/4/5 required only CD and that can be emulated by Dosbox), GNB1 had three: main game, scenario player and scenario editor.

GNB1 main game (GNBNA386.EXE) has no issues, it simply works.
GNB1 scenario editor (SGEN.EXE) has no issues, again.
GBN1 scenario player (GNBSP386.EXE) has issues and quite a lot:
1. it is unpatched, so it asks for copy protection questions (OK, those can be found in the manual, but still...)
2. it simply "freezes" on certain screens (I am not sure, if it just a loss of keyboard/mouse control or if it really freezes) - it could be this version, it could be GOG's Dosbox, I simply don't know at this point.

Interestingly enough, my old GNB1 copy works normally, I can run it in Dosbox with no hassle at all...

Just FYI, in case that someone has some ideas.

I will contact GOG support sometime next week in this regard (no point contacting them right now).