So how can I get stable 2.15?
I'm not interested in beta testing but all GOG gives me to download is experimental build.

Can't help you with 2.15 as the off-line installers now have 2.16. Just download Parts 1 - 4 and ignore all of the patches.

To stop getting Experimental you need to change your settings and channels in Galaxy.

Launch Galaxy > No Man's Sky > Settings > Beta Channels and select Off. Before exiting select Channel and set that to either Default of turn off beta channels.

Note that if you have an Experimental version already saved then you probably will have to redownload the entire game.

EDIT: Also disable automatic updates in Galaxy to prevent any changes being made unless you specifically allow them.

Thank you for your answer!

However, I don't use galaxy and I can't download full 2.15 installers.
I find this disturbing to be forced to play beta branch without option to play stable release (or forced to use Galaxy).
This is not what I paid for, dear GOG.

Ah now I understand .. The naming convention of the Offline Backup Installers is wrong.

I think they just declared the latest experimental as stable, and made those files directly into what is version 2.16 ..

But they did not change the names of the files.

They are called 2.16 before you download them on the NMS GOG Game page, but the files you receive are not named correctly, and you believe you have the Experimental version as offline back installers

What you actually have then is the fies for v2.16, named incorrectly ..

setup_no_mans_sky_experimental_53949_(33730).exe
setup_no_mans_sky_experimental_53949_(33730)-1.bin
setup_no_mans_sky_experimental_53949_(33730)-2.bin
setup_no_mans_sky_experimental_53949_(33730)-3.bin

These are the snapshot files which were made into v2.16

Note the number in the name of the above files 53949

Is also the number on the files you click to download

See screenshot attached

The most recent Experimental build is 54016 (the one which has been on GOG Galaxy all weekend with just one fix)

Thank you!
This explains everything.
Support didn't confess in breaking the names and added more confusion.