Hi I have Serious Sam Fusion installed on Steam for all the other games in the series, but it doesn't detect Serious Sam 4 (the only one I have bought on GoG).

How do I make SSF detect my install of SS4, so I can install mods from the Steam Workshop?

Serious Sam Fusion does not support Serious Sam 4, even on Steam. And since Serious Sam HD and BFE are not on GOG, I don't think Fusion will be coming here, but who knows.

The game still doesn't have the Workshop Steam, let alone here.

I honestly have no idea what will happen once they get it on Steam. Will GOG be left in the dust? Most likely I'd say. Unless the mods are made available for download manually that is.

And Fusion will probably not come here. Croteam was and still is pretty heavily entrenched on Steam. The fact that we even got SS 4 here was very surprising. Maybe they will change their mind in the future but right now, it doesn't look like it.