I heard the GOG and Steam version of this (especially SOTS 2) are not compatible because GOG is behind on the version.
From what I heard the Steam version is 1.8.1.
Can anyone confirm this?
What is the current version on GOG?

It's version 1.8.0.

Thanks. Turns out I had bought it already. The steam version is 1.8.1. I contacted GOG support and they said they'll look into it.

GOG is now 1..8.1 too according to gogdb https://www.gogdb.org/product/1450279740#changelog

Me and friends would love to play Civilization IV together. Alas, some of us got GOG, while others Steam version and they don't seem to be compatible (LAN does not work and online play of GOG version is disabled).

Please fix.

I've noticed that incompatibility with games like this from GOG and Steam may exist. I got particularly pissed when I was unable to play online with friends using the Steam version of Stardew Valley and Terraria.

Does anyone see this as an anti-competitive practice on the part of Steam?
As far as I'm concerned it very well may. Exempting technical issues it seems like Steam restricting compatibility of products purchased through their service.

Anyone willing to see if this can get called out by some clever lawyers in an anti-trust suit?

Might be worth considering.