When does version 0.15 appear on gog?
Not Beta.

Presumably when 0.15 is stable and the devs release it. As of 22/07/2017, 0.15 is still experimental, although it's pretty much as good as finished. In the meantime, you can go to your account and download the experimental version, or switch to the beta branch in Galaxy. 0.15.30 is very stable and a lot of people are already using it.

I am a little bit confused, because on Steam they released some news about 0.15.30 as final version and facts or ideas about 0.16.
So my thought was they already released 0.15 as final.
At the moment I am not interested to play the Beta.
But thanks for the hints.

steamcommunity.com/games/427520/announcements/detail/2383944632764371381
steamcommunity.com/games/427520/announcements/detail/1429182063902080414

stable is 0.15.31. its been announced on their forums. they no longer support 14.

See dev notes: https://forums.factorio.com/viewtopic.php?f=3&t=51241

15.31 is now the latest stable version. I assume you had played 0.14 previously, you can role back to that version by choosing the beta 0.14.23 in steam, but the 0.14 will not receive updates anymore so you will have to switch to 0.15 eventually. If you need further info please open a thread in the tech help section.