Not quite. You can use Galaxy to download the offline installers (or just do that through the website), but when you install a game through Galaxy, it downloads a special build which it just copies to the right location, without running a separate installer.

No, it doesn't use offline installers. Source: worked on that in the past.

https://content-system.gog.com/

The running game can also call out to Galaxy and unlock, or not unlock, ingame content based on what it hears back. It's pretty difficult to imagine a definition of "digital rights management" that doesn't include this.