Yes, Galaxy is optional, you can download offline installers for the games (click on your account name on the upper right -> Games -> click on the game you want -> Donwload offline backup game installers).
When you've downloaded the files, just run the setup file.

Just check these Free Games then you will see how them work in the same way when you purchased a paid game :)

Some games will require Galaxy for multi-player, but otherwise can safely be used without it.

Some games like GWENT require it. While the catalogue is incredibly small right now, the amount that will require it later is unknown. I recommend searching other stores like itch.io, dlsite, jast, Zoom (there's a game site unrelated to the VoIP product), etc before purchasing here, since other sites are more reliable at stating what does and does not have DRM in them.

For example, The Witcher [Enhanced] will appear in your account with all the particulars listed (OS, Language, a drop-down selection for the various resources like the changelog, store page, sub-forum, etc.) followed by a big blue button to let Galaxy download the game into your system. Beneath all that, as noted above, is the heading DOWNLOAD OFFLINE BACKUP GAME INSTALLERS. Click on this heading and four lines will appear beneath it, as so: When you download them, they will have a different filename, e.g., the first (1MB) file is currently named:
setup_the_witcher_enhanced_edition_1.5_(a)_(10712).exe

Each of these files is required. They must be downloaded to the machine you wish to install the game on, or the game will not install. (The installation is triggered by running the first file —— the only one that is an .exe executable, all the others have incremental .bin suffix. The installation will ask where you want to put the game, so select the folder and check the box about terms & conditions and then press start.) The installation process will do an integrity check of the downloaded files (making sure none are missing or corrupted) before the installation proper begins.

That's it.
:)

Besides what's been told already, it's always a good measure to keep back ups of the files you get here.
Unlike Steam, where you are just renting the titles, one of the ideas of distributing DRM free games is that it's yours to keep.
Also, some updates might come borked, so while things are not patched, you either need to have the installer of a previous version or use the Galaxy client to download the old builds.
And while rare, there have been cases of games being pulled from people's libraries due to bugs or because the devs said so. For this case of devs asking for games to be pulled, GOG staff says this situation will never happen with paid games, but while I hope that stays true, you never know what tomorrow holds. And since stuff here is yours to keep, in theory, you just need something to prove you own that title (e.g. purchase receipts).