any plan on that?

Never has been available on GOG, and GOG users, aren't really rabid fans, demanding to play the second a game releases.

Sensing a pattern to your posts, which is unspoken but clear,
"Why isn't GOG just like Steam?

Main reason - Steam is much more popular, due to it's practical monopoly, and gets preferential treatment, that goes a long way towards maintaining that monopoly.

Secondary Reason - The core GOG user base, doesn't care for such things, many refuse to ever preorder a game, so don't need to preinstall it, most of us wait for reviews first, and SpellForce 3 has no Full release reviews out yet.
These are also new devs, and publishers, that have bought the IP, in a bankruptcy sale of THQ owned IP's.
This makes us highly suspicious about the games quality, so we wait, and see.