It seems that you're using an outdated browser. Some things may not work as they should (or don't work at all).
We suggest you upgrade newer and better browser like: Chrome, Firefox, Internet Explorer or Opera

I noticed how the Steam page mentions Online COOP, where in GOG it mentions singleplayer only. So it likely uses Steam for multiplayer.
Question is, will this change in the future to allow GOG multiplayer or even crossplay?

Secondly the OST is also only available on Steam, which is just weird. I don't see a reason why it couldn't be also distributed through GOG.

I love to get it on GOG, but this lack of feature parity makes the steam version a much better value for the same money.