What type of multiplayer the game will support? Local, online, will it require the Galaxy client or will be playable on LAN? I'm curious because the game looks promising so far. For me the more direct connection was always the prefered. If one can host and the others can join with IP, that is perfect.

I want to know multiplayer details too!
some good news after quick search on steam forums : crossplay steam<->gog is confirmed
link

This is a kinda edgy answer since it may mean:

-There is only a single server software and the game runs through the platform's clients to connect to it.
-The game will not feature platform hosted servers but it is DRM free at core and capable of connecting to each other with IP.

The second would be the better by far.

do you have galaxy installed?
because devs said on steam forums that lack of galaxy support is the reason gog doesnt have linux version.

Thank you for the detailed answer. Seems like it worth a try.

I have both game in steam and GOG and I confirm you cannot create a private game and play cross platform...

There is no option in game like there was in Hammerwatch to join via direct IP.

Also there is no option to play local couch coop

This is a real let down since I rather like to buy it from GOG and my friends are mostly on steam...

lack of private lobbies is notorious "feature" of gog/steam crossplay - I "enjoyed" it in Shadow warrior 2.

But in this case seems like there is a solution - you can use console in game (~ key) - and if you are host you can try command g_multiplayer_limit <number>
People saw 20-man lobbies haha. you can simply put 2 players limit on yor lobby if you want to play with friend

That is bad. I wish it will change, because then it is a big no from me.