DRM-Free is not just one type or category as some here seem to think.
Like most things in life there are variants and degrees. Only the pedantic think otherwise.
So we have DRM-Free, with no DRM at all.
Then we have DRM-Free, where only the single player aspect is DRM-Free, the MP aspect sadly not.
Then we have the variant known as DRM-Free Lite, which at the very least requires backing up a game folder, and possibly doing something else usually simple and legal.
They have commonality, in that all those games can be played when and where you like, without your digital rights being managed by anyone other than yourself.
What you use to gain access to those games in the first place, is really neither here or there, because they all require verification that you own the game before you can download.
Yes, the GOG variant, with or without Galaxy, means you download a ready-made installer ... talking Offline Installers.
When it comes to Steam and Epic etc, you get a game folder, which most of the time you can just back up, and if smart ZIP, to reduce storage size, and if you make self-extracting, your own variant of an installer ... referring of course to the DRM-Free Lite variants..
Steam gives you two ways to get your game - (1) GUI or (2) CLI client. The CLI called SteamCMD has benefits over the Steam Client GUI version, in that it is much quicker and allows you to choose per game, where to install it. Quicker in that it updates itself quicker than GUI version. Also much lower on system resources.
For Epic, you get a somewhat similar situation if you use the third party CLI program Legendary or the Epic GUI client.
One can argue until they are blue in the face, about the specifics of what really constitutes DRM-Free, but for me it is all about the level of control I have over a game ... that's the thing that is the most important, at least to me.
The next most important aspect, would be how much trouble I am willing to go to when it comes to DRM-Free Lite.
And of course, DRM-Free Lite isn't guaranteed, so it can be about taking a gamble as well. Naturally, once you've backed up your game and certified it is indeed DRM-Free, then you are laughing I guess.