Alexim: I was able to contact the developers via Twitter and received an encouraging response about the GOG Galaxy achievements: "Hello, we will try to handle it, first GOG launch for us."
I think as many people as possible should try to contact them, because the developers are not active in this forum and have no idea how much demand this feature really has. This is their official Twitter page and you can contact them privately by clicking on the envelope symbol:
https://twitter.com/_FQteam I also asked about the soundtrack and they have to ask their publisher, Devolver Digital, because they are in charge of it.
Malvictis: Thanks for the info. However I believe messaging the devs on Twitter is an option exclusively to those who use the platform (i.e. no DM option without an account). I think it's a sad state of affairs when the GOG customers have to go out of their way to ask the devs/publisher for the same benefits as those on Steam...
Also, not only do the devs completely neglect the GOG customer base but, after going through the newest post on their Twitter account, they seem to be busy adding even more stuff for the Steam users with the
new Steam trading cards. Ridiculous.
I agree, I myself had to create a Twitter account to contact some particularly elusive developers. Unfortunately this is the state of affairs and there is not much choice.
GOG should be more strict about the completeness of their releases, but at the same time they may risk alienating developers, so I understand it's a fine line to walk.
The only way is always one, hope that more and more people buy on GOG so that it becomes a more and more important store that developers and publishers must take seriously into consideration.