Hi,
Sorry for the delay, I don't look at Itch comments often.
Though I have never done it, you should be able to use Gog's steam compatibility API to create a release, you will just need to setup a specific Ren'Py install as you wont retain the ability to create a steam build.
Long story short:
- Install Ren'Py in a new location
- Add the Steam SDK to the new Ren'Py install
- In your file browser, go to where you have installed Ren'Py and replace the "libsteam_api.*"/"steam_api" files with Gog's steam libs (and add Gog's lib file to the directory)
- Update the file "update/current.json": Go to the "steam" section, and add any additional Gog SDK file to the list of "files"
- Build your game with this modified Ren'Py install