Hi
I've manually tag all my games, adding missing pictures and renaming some game names.

But is there a way to backup all this metadata, and to import it on a new GOG install ?

Same question here. I'd hate to lose all of the many hours of work it took to meticulously tag and organize my insanely large collection. I installed my GOG Galaxy 2.0 Client to a Dropbox folder just in case something bad happens...This in theory would make it portable by re-installing a base copy and overwriting all contents from Dropbox. Not sure how this would handle client upgrades though, so would probably need to restore to the same version and then update.

I would love to see a one click backup\import library feature!

There is the %ProgramData% Folder if you are on Windows (don't know where it is on MacOS) and there is the synchronization part with gogs servers(excerpt from the pinned FAQ):

What gets synced with gogs servers

- Custom cover arts
- Manually added games (you will need to re-link the games once in a new PC or installation, though, but they're still on your list)
- Games you own on every platform
- Tags
- Edited metadata
- Hidden games
- gametime for manually added games

What does not get synced with gogs servers

- Bookmarks
- Integrations