Yes, why can't GOG provide those for Linux versions as well?

Here's a weird guess: Since Poland was formally behind the Iron Curtain and all software was state owned, they want to have a sense of accomplishment and pride in creating something themselves and owning it wholly, opening the source would just remind them too much of the times of yore.


And here's my rational guess: As much as they'd like to make it open source, they'd need someone or several someones to vet the code, and a licence that would allow it to be used commercially, as GOG Galaxy would allow them to make money. But if the community were to make it in Rust, Ruby, or RuneScript, while in house GOG specialized in MIMIC, LANSA, and COMAL, that leaves something of a language barrier.

Nothing stops them from using it commercially, I don't think any specific FOSS license prevents that. The only ones that can do it are something like Creative Commons variants, but they are not used for code normally.