Solved, kind of.
I was using:
- Greenworks 0.71.0
- NW.JS 0.71
- Steam SDK 150
And no matter what, it reported an "Uncaught Error: the specified module could not be found"
Which as I found is a catch-all message for many kinds of errors. I re-checked that every file was in the correct place and I tried downgrading to Greenworks/NWJS 0.60.
When I tested again it reported a different error: "Steam_appid could not be found" even though I placed it in the base folder where the .exe for the game is found. After copying it to the package.nw file it worked, and the previous error was gone too!
My game seems to be working fine with the downgraded version, but It's weird that it won't work with the updated ones.
Anyways, solved.