Hi guys!

I'm playing Sunless Sea on a Humble Bundle DRM free version.
Normally, I'm buying on GOG, but there was a good sale at that moment.
I want to buy Zubmariner, but on Humble Bundle it's listed as only Steam.

Do you think that I could use my Humble Bundle DRM free vanilla game with the expansion from GOG?

Thanks!

Really not sure.

I would contact GOG support and ask them directly.

In theory there should be no issues with this, especially since they're both DRM-free; however, there's no guarantee it will actually allow you to install the expansion, since GOG installers for upgrades may check for the existence of the base game (which assumes it was a GOG version, so may not detect a legitimate version from somewhere else).

One would hope that these days GOG installers don't do those kind of checks anymore, or at least do them in ways they can still be used with the game acquired from elsewhere - and if not, that GOG does something about it to make it work...