Make sure you have the following file structure (am assuming you used the default location installation from GoG)

Program Files > GOG Galaxy > Games > Stardew Valley

in the Stardew falley location, in addition to the gog installation files and the game executable/ ini files you need to place 'StardewModdingaPI.exe' file. and the directories /Content & /Mods (containing the mods you're trying to run)

The way to actually get this all to run --> instead of using the GOG galaxy buttons, create a shortcut of the 'StardewModdingaPI.exe' and place on the desktop - run THAT to play the modded game.

This is kinda the problem I have. I like the game, I like the modded game. I would prefer to run the modded game from inside GOG galaxy (and have them keep track of my hours). but I can't get that to work and haven't heard that anyone is woking on that aspect :(

The last post might help if I understand correctly what your problem is.

whohooooo! that worked great! THANK YOU!

I never would have tried to edit any of those files without some kind of clue. Now I can let GoG keep track of my time in game (I've maybe a hundred hours in Stardew valley, but GOG recognizes about 20, I think.)