I feel that this is a pretty important topic to discuss, especially now in light of GOG's latest stance on Game Preservation.

Using Galalxy, you can get your game from GOG to your PC in two very different ways.

(1) Offline Installers - Available via the Extras menu option in Galaxy.

(2) Direct Install - Galaxy downloads all the files & folders required to an install folder or pre-install folder.

I believe there was a time, when Galaxy used to just download the Offline Installers to a temporary folder and then automatically execute them on completion of the download. Now however, it streams all the parts of a game, either as compressed files to a pre-install folder, from which it then installs decompressing to the install folder, or the streams are already uncompressed, and go straight to the game install folder. QUERY - Someone here probably has the knowledge to clarify for us, which process actually happens.

Offline Installers
These are a prepackaged and compressed version of the game, ready to install, and come as one or more files, depending on the size of the game. You will always have an EXE file, and if the size of the game warrants it, it will also be split into one or more BIN files. The EXE file is the installer, and the BIN file(s) are the data. This is a ready made portable version of the game, easily backed up to external drives etc. Thus we can call it fully DRM-Free. NOTE - GOG use InnoSetup to create Offline Installers, and InnoSetup usually splits the result into 2GB parts, but GOG have an arrangement, where their version splits it into 4GB parts instead.

Direct Install
This results in a fully installed game in a game folder. At this point we have to call it DRM-Free Lite, because it isn't compressed, and it isn't a very portable or installable version of the game. It may also have some kind of dependency, that unless clear, makes the game DRM rather than DRM-Free Lite ... this could be Registry entries and or files installed outside the game folder.

However, in most cases the game install folder can be backed up as a DRM-Free version of the game. Being uncompressed, it is a much larger copy of the game, compared to the Offline Installer(s) version. That can be improved though, if you zip that folder up, which also improves its portability. You can even go further, if for instance you use 7-Zip to make it a self-extracting installer. 7-Zip can also split the resulting files into an EXE and data files at a size of your choosing.

In this way, so long as there is no dependencies, we are somewhat matching the official Offline Installers, except that when executed, the Offline Installer(s) version also runs an MD5 check of the packaged files during install. The differences are why we call it DRM-Free Lite, and unless you know otherwise (having checked or been told), might not be fully DRM-Free.

It would be interesting to know, how long it takes with both versions, to get a game downloaded to your PC, and the difference in bandwidth. Of course, Offline Installers might be deliberately set to download more slowly.

Game Preservation
This is not something GOG can do on their own, chiefly because they could disappear any time, and game files need to be distributed far and wide, to be truly preserved. In essence, GOG enable us, their customers, to be the true preservers of a game.

So one needs to consider Offline Installers versus Direct Install, when it comes to Game Preservation.
We need to ask ourselves a few questions.

(1) Is a Direct Install game folder truly DRM-Free?
(2) Are many gamers who use the Direct Install method, going to backup the install folder of a game?
(3) How important is the MD5 checking during install?
(4) Without the MD5 check ability, how sure are we and how safe is our DRM-Free Lite backup?
(5) Without that surety, is the game truly preserved?

Of course, there are some gamers, reputedly, who do the Direct Install method, plus also download the Offline Installers and back them up. That seems crazy to me, but then for those with a fast connection to GOG, probably not much different to backing up a game install folder and zipping it, time wise ... and you get a better result.

One can foresee a time perhaps, where GOG do away with Offline Installers, if just to save on server space. They could offset that, by giving Galaxy an option to compress and backup a game folder for archiving. This might be acceptable, if it also ensured that any dependencies are catered for, plus adding an MD5 check facility as well, would be the final important element. They could then claim, that Galaxy can create Offline Installers.

We don't know the future of GOG or indeed DRM-Free, especially in a world that has always been more concerned about DRM and very active in that concern. Game Preservation though, ultimately and clearly relies on DRM-Free.

NOTE
There is a slightly different scenario for Linux and Mac versions of games at GOG, which are predominantly Windows based. For starters, InnoSetup is not used to create them, and we have SH files (Linux) and DMG and PKG files (Mac) instead of EXE and BIN files. And usually, but not always, Linux and Mac files are not split and are just one Offline Installer file download.