**merged with existing topic with a similar question**
As Lander1979 mentioned not all mods are hosted in the Steam Workshop. Due to various reasons people have used other hosting sites such as nexusmods or dropbox instead. If the creator of a mod is still active, you can also ask them to upload the mods somewhere else with the hint to the problematic nature of downloading them for a GOG installation. If you cannot find any alternative download for a mod at all, then with the
Steam Workshop downloader and Lit's
XRWSunpack tool you can download and extract mods from the Workshop without having Steam.
As for the tools, I will ask, whether they can be made available outside of Steam.
Edit: It's definitely on the agenda to make the tools available somewhere else than Steam too. Unfortunately, I don't really have an estimated date for you when this will happen. But I'll keep you updated, once there are any news regarding this.