This is available freely to steam customers !

Does it mean that in the future it will be better so buy somewhere else ?

That's exactly what I was thinking. I know it's not really Gog's fault because it's up to the developers, but it is unfortunate, Especially with PDX's game pricing strategy. It did just come out, so perhaps in time, couldn't hurt to mention it to the dev so they know there is an interest.

it's still in open beta, I sincerely hope that when they'll release the stable version they will also publish it here...

(I generally don't test betas so I'm not interested in the open beta)

Here: https://steamcommunity.com/games/859580/announcements/detail/3719452816826633917

"[...]
As you can see, this is a minor patch, and for now it will be released as an open beta on steam & windows only. The plan is to release this on all platforms at some point in the undetermined future. Please leave feedback in the comments section, I am quite bad at replying, but I will read them.
[...]"

(in the announcement then there are 39 pages of comments that I haven't read, I don't know if something else has been specified about the stable version)