How can share custom maps?
Please help.

I am interested in this option too. The trailer for the Steam version contained information about Steam Workshop support, whereas the GOG trailer contained information about GOG Galaxy support. Does it mean that custom levels can be somehow shared via GOG Galaxy? If yes, then I will gladly learn how to do it. Is there some way to access such a repository?

I would like to learn that to unlock the "Pillar of Community" achievement. I tried unlocking it by creating and then playing some local maps, but it did not work, so I assume this has to be done differently.