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.

Really? What games are those? I've not encountered a single one :/

Many games with multiplayer features require Galaxy for those multiplayer features. You can consider this DRM-equivalent if you want. However, every singleplayer game on GOG will work without Galaxy installed, and that singleplayer gameplay will be completely DRM-free in every possible way. (That's at least 99.6% of the games on GOG, but eyeballing the 22 games which don't specify that they're singleplayer games, most of them simply have incomplete metadata, so it's really 99.9% of them.)