There are a couple of DLCs for OTC, that GOG doesn't offer. At first, they didn't seem interesting, so I didn't care.

After playing OTC for over 20 hours, I really getting into the game, can see I'm going want more content (already have 3 DLCs for this game).

Offworld Trading Company: The Europa Wager Expansion: This one would be a likely next choice ... if GOG sold it. But can I buy somewhere else, and still play it with my GOG copy of OTC?

You cannot redeem DLC bought at another store with GOG unless it's specifically for the GOG version.

If you buy the DLC on e.g. Steam, you will also need a copy of Offworld on Steam. At which point, you might as well play the entire game on that platform directly.

The game should be removed from sale if they are not going to bother to sell the rest of the DLC here.