I actually bought Majesty Gold HD on GoG because I had heard about the modding tools. Then I found out only the steam version has them.... grrrr. I then went and bought it on steam just for the modding tools. I am not sure if the modding tool (or mods made) will work with GoG version. The steam version of the game has a "beta" version option for the game and I think mods and the tool will only work with that version. I really really wish they would release the beta version for GoG.
Aye, a wee bit o' necromancy there.
Also it may be possible to do some modding even without the beta version. The SDK come with files that can let you replace the games .bcd files, so you can make changes that way (fairly sure they would work in non beta... but don't quote me).
I'm in a similar position. I heard about these mods so came to downloaded majesty and try them out . . . . but now I see they all seem to be for steam. I've only ever owned games on GOG and have never used another client . . . . but having mod integration is something that could make me switch. Especially for games like majesty which could benefit alot from a little modding attention.
Have you tried getting any steam mods working?