Hi,
on Steam there is access to what is called 'Experimental' version of NMS which is released by the devs so players can help them fixing bugs.
Can I access this version on GOG, with GOG Galaxy or another tool?

Thanks,

Yes... Yes, you can.
Same password as for Steam.

Thanks for your response Gyrofalcon, but please could you elaborate further (without giving the password ofc) ?
What do I need to do in GOG Galaxy to access?
Thanks,

The password is public knowledge but I don't use Galaxy, so I couldn't tell you where to enter it. The password is "3xperimental", IIRC. Don't use the quotes.

See this thread for further elaboration:
https://www.gog.com/forum/no_mans_sky/crashes_on_pc_when_opening_galactic_map

Current version of Experimental Branch is the same as the standard game. In fact standard game v1.34 is newer than Experimental, as it came later. Dunno if it is fully the same as Experimental b1.34. I would advise to use standard for now.