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).

to start the editor for gnb4:

I.
create file "run-editor.GNB4.bat" in \gnb3-4
content of this file:
@start "" /W /B DOSBox\DOSBox.exe -noconsole -conf gnb4-base.conf -conf gnb4-graphics.conf -conf gnb4-editor.conf

II.
create file "gnb4-editor.conf in \gnb3-4 diretory
content:
[autoexec]
@echo off
mount C GAME
imgmount D GNB3.cue -t iso
config -securemode
CLS
C:
CD GNB4
..\INT0MOVE.COM
SEDITOR4.EXE
exit

III.
execute bat file

I'm researching this a bit. If you don't call the patch gnb1mse.com (i'm not sure what it does), it seems all versions cd/floppy, main game, scenario editor and scenerio player works fine

In my case I used a config file like this for DOSBox Staging.
I can play now easily captains mode too!
I place a DosBoxStaging folder inside each root folder of each GOG game, and I use it instead vanilla DOSBox to have a better sound, resizable windows and other features



[sdl]
mapperfile = mapper-staging-1.map

[cpu]
cpu_cycles = 8000

[midi]
mididevice = mt32

[mt32]
model = mt32_new

[autoexec]
# Each line in this section is executed at startup as a DOS command.
@ECHO off
cls


mount C ..\GNB1flop\Game
C:
cd GNBNA

goto launcher

:launcher
cls
ECHO ------------------------------------------
ECHO Great Naval Battles I (floppy) Launcher
ECHO ------------------------------------------
ECHO 1) Play (Campaign)
ECHO 2) Play (Captains + Custom)
ECHO 3) Editor
ECHO 4) Setup
ECHO ------------------------------------------
ECHO 5) exit program
ECHO ------------------------------------------

choice /c12345 /s Which program do you want to run? [1-5]: /n
if errorlevel 5 goto exit
if errorlevel 4 goto setup
if errorlevel 3 goto editor
if errorlevel 2 goto gamecc
if errorlevel 1 goto gamecm

:gamecm
cls
rem ..\gnb1mse.com
call GNBNA.bat
goto exit

:gamecc
cls
rem ..\gnb1mse.com
call GNBSP.bat
goto exit

:editor
cls
sgen.exe
goto launcher

:setup
cls
sound.exe
goto launcher

:exit
exit

Great!

Honestly, I have been lately so busy, that I had no chance to tinker with anything... does your batch work with CD version as well? (I, know, it lacks music and most of the sound, but that what makes it appealing for me - and no stress I can test it for myself :-) ).

Anyway, Dosbox staging works fine indeed.
Few months ago I was pinpoint the culprit of my issues with GNBSP and it is the Dosbox version provided by GOG.com.
Other versions of Dosbox, either vanilla of whichever SVN build worked just fine even with GOG version of GNB1.
After all, I was able play Captain's game in Dosbox in Windows 10 several years ago - however, my CD version still requires copycodes at every turn...

Which begs another question - how is the situation of GNBSP now? Does it still require copycodes?

Once more, thank you for job well done!