Mr.Mumbles: When Mama Freebies and Papa Freebies love each other very much...
Came here to post this :)
Though seriously, I believe I asked this question before years ago and the general consensus was it depends on the situation.
Out and out freebies (like Full Throttle in this sale) are probably a deal with the devs that benefits both parties, if the devs are promoting something they might let GOG give them away completely or they might charge a small fee per game (let's face it, it doesn't cost anyone anything for these freebies so it's not like there are expenses to cover). Because of the (presumable) bulk involved even if GOG only gives them £0.20 a game they might still make quite a bit.
For the ones you earn by buying I imagine GOG negotiates a discount price same as if they were going on sale and then pays the dev this for each one given away (again, unless the devs are particularly interested in the exposure gained, which won't be as much in these cases, then they might give them a further discount).
If the devs are willing to sell their game at 70% - 80% off then once you also remove GOG's cut you're looking at between 14% and 21% of the original price.
In the case of Everspace that would be around £4.83 or less per game.
Now GOG should make about £3.54 for each £11.79 spent, but most people will probably spend more than £11.79 and it will only really be the bare minimum who just spend enough to unlock Everspace.
Fantasy General is a different story as GOG owns it, so they can give that away as much as they want, so the total cost for each copy of Everspace might be anywhere between nothing and £1, which means they may not make any profit on people who only spend £11.79 in the sale and only start making profit after that point.
But considering GOG's game is still to build a user base then taking a hit to get more people owning games and buying games here is worth it. And that's assuming they haven't negotiated an even better deal so they are still making some profit (which I would imagine they have)