aye, the biggest turnoff of any game.

Which is why I can't understand why you like RTS. In every RTS game I've played, there always seems to be a limited amount of time for base prep and army building before the first wave of enemies hits you and then they keep coming until you win or you get curbstomped. You don't get to ponder and think about your next move like in a TBS.

That depends on the game.
There is always a siege level or two, with a countdown, but games like AOE, AOE 2, StarCraft, C&C tend to give you all the time you need more often then not.

Games like Dark Reign and others don't. From the outset you are under attack, even on easy.

ALSO, I DO NOT play an RTS without Pause & Play.

Hidden Objects Games
12 Labors of Hercules Games
Racing Games

Is that a 100% true though? I was under the impression that time also passes when you don't move, just very, very slowly. Then again, it's been a while and maybe I just wasn't able to keep my fingers from the keyboard and stand completely still?

EDIT: Hm, looking at some gameplay footage, I have to concede it's probably the latter ...

These two came immediately to mind when I read the first post: Been meaning to install and try: Should give it a whirl this week.

No, as far as I remember, there is indeed a slight, very slow, movement even if you're immobile. You can't leave your keyboard with a bullet about to hit you, go get a coffee, and come back to find yourself alive.

(It makes sense in context, I swear.)

But yeah, Superhot is an excellent suggestion.

Superhot is definitely a great suggestion! We actually would play this with a VR setting in the barracks common room. It was a lot of fun!
These are actually really interesting suggestions. They fit in the category really well IMO. Obviously for more casual time killers, but I hadn't even considered them.

Zelda 2 (not on GOG, of course).

I remember, after being used to more modern Zelda games like Link's Awakening and Ocarina of Time (*especially* the latter), coming back to Zelda 2, and it felt like I made much more progress in any given amount of time than in the more modern games. Zelda 1 would probably qualify as well.

I notice a similar phenomenon in the Metroid series; the 2D entries are pretty quick, but the 3D entries are a lot slower.

In terms of games that are available on GOG, I could recommend Timespinner; the game plays pretty quickly, and cutscenes are (for the most part) skippable.

(With that said, I would avoid early RPGs if you're looking for games where you can make progress quickly. (Keep in mind that my definition of RPG doesn't include games like the Ys series; Ys I and II might actually qualify for this list.))

The twin stick shooters Crimsonland* and JYDGE are good for that, too. One level only takes a couple of minutes (even just one or two sometimes) but they're fun and addictive.

Depending on your skills and frustration tolerance: Hotline Miami,* VVVVVV,* Super Meat Boy. You will most likely die a lot, but you can jump right back into the action again, and if you have to ragequit, you will hardly lose any progress, as levels are short or checkpoints are everywhere.

Or fast paced action side-scrollers with shorter levels like Broforce* or Not A Hero*. Maybe Gunpoint or Ronin,* if you prefer them to be a bit slower and more tactical.

Or simple card games like Card City Nights* or Guild of Dungeoneering*.

Or puzzle games that are also suitable (maybe even developed) for mobile devices, like World of Goo* or Tomb Raider GO.

Maybe it's even better to play all of the games mentioned above in short bursts as they might get a bit repetitive otherwise.

* available on GOG

If you're going to mention those games, you should also mention Celeste (not available on GOG, but available DRM-free on itch.io). The second sentence in the quote applies to Celeste as well. (This is assuming you don't go for the golden strawberries, but then again the VVVVVV suggestion assumes you're not going to try No Death Mode.)

This short (3:46) video review sums it up pretty good:
What is Bad North?

Here's a slightly longer one (6:45) covering what Brigador is about:
Brigador Review
I STRONGLY recommend picking up the Deluxe edition which also includes the outstanding retro synthwave soundtrack and an audiobook version of an original novel set in the Brigador universe.

Regarding Battle Brothers I found the tutorial videos by the devs, Overhype Studios sufficiently informative:
Worldmap Tutorial
Combat Tutorial
First 37min of the campaign
There's also the Supporter Edition Upgrade if you feel like showing extra support.

You could try Binding of Isaac: Rebirth. This is probably the only game I have ever played with so much "time" value as you put it. You can play it in short bursts & get lots of fun out of it. Also, if your time is as limited as mine nowadays, then you can probably keep playing it forever since there will always be something to unlock (100% replay value due to randomized dungeon layouts each time & huge amount of unlockable items/abilities/weapons/etc.).

PS. I know it is not available on GOG (unfortunately) but I assumed from your post that you also buy games elsewhere.

I haven't played Rebirth (don't own it), but the original Binding of Isaac is definitely not suitable. One complete playthrough is going to take more than half an hour, and there is no way to save in the middle. (Yes, there are speedruns that are faster, but they generally involve getting extremely lucky and getting a very powerful (often gamebreaking) combination of items early on.)