alexandros050: Yes DRM Free means no internet connection , no activations etc. It is yours to keep forever, you can back up your games on a usb or external hard drive. You just install the game and play it...

If you can also post your computer specs because some new games might have problems if you have an old computer.

Maybe he would like the witcher series? The witcher 1 that even old computers can run.

TheMonkofDestiny: The Witness, assuming what he loved about Myst was the puzzles + setting. The Myst series is also available here but if he was a fan then he's probably either got actual copies of them or has already purchased them digitally from here or elsewhere.

zeogold: Your brother is a man after my own heart. Tell him I said this.

What he's looking for are games with environmental puzzles. If he likes Myst, there's a whole series of Myst games afterwards, starting with Riven.
Quern is absolutely fantastic, I'd definitely say get him that.
Firewatch isn't quite the same thing from what I can gather, but I haven't played, so I can't confirm.
Obduction is made by the same devs who made Myst and has that same puzzly goodness.
Schizm: Mysterious Journey is a decent Myst clone.
If you're willing to go outside of GOG, look into the Rhem and Aura: Fate of the Ages series.
Amanita Design's games (Machinarium, Botanicula, the Samorost series) are rather similar in the "figure out how the world works" sense. I recommend 'em, though it's not the same type of first-person view.
If we get more into stuff where you start to have an actual inventory, some of Kheops Studios' games are nice, like Return to Mysterious Island 1 and 2.

DRM-free means basically what you just said. You get the installer, and there's no extra steps you have to go through, you can download the game and install/play without internet. If you'd like a client (to manage your game library, a bit like Steam), you can download Galaxy. If not, just use the "offline backup installers" when looking at the game in your library.

Microfishd: I must confess to having never played MYST, but if he likes strategy games he might enjoy Civ 1, 2, or 3. He also might enjoy Railroad Tycoon 2 (those were my favorite games for many years).

HunchBluntley: Is his PC a laptop, or a full desktop setup? Because if it's a laptop that's old enough to have shipped with Windows 8 (or older), then unless he specifically bought a souped-up gaming model, it might not meet all the minimum system requirements to be able to play all the more recent first-person adventure/puzzle games (such as Firewatch and Quern). Older games (such as most of the actual Myst series, which are now all available here) should be fine, but even with those, you should still check system requirements (and any associated notes) to make sure there aren't any hiccups, such as a particular title not working on multi-core CPUs, or not being supported on Windows 8/8.1.

Edited for clarification

Bohemian59: LOL, he is retiring from 35 years on the Railroad!! Thanks for the suggestion