I don't have much time lately, so I just want to play a very short game. What are the shortest games on GOG?

Assuming you also want a good and modern-ish game, probably the original Kingdom.

edit: oh wait, it's definitely Gorogoa.

To contribute more to the thread, there are some good finished (pseudo-)episodic games:
The Darkside Detective
The Last Door and part 2
Mainlining

For stories driven games, To the Moon is about 4~5 hours.
For stage based games, it could be as short as 30 seconds per stage.

It's a strange game. The only way to win is not to play.

Akalabeth is a nice short game; in fact, the game did not originally have a save feature. (I am not sure if the GOG version has one, but I know the version on the Ultima Collection CD does; note that the save feature breaks the game, but so does RNG manipulation.)

Also, the game, at a price of $0.00 (or the equivalent in your currency), shouldn't break your budget.

(With that said, this game is definitely not modern; it might even be the oldest game on GOG.)

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Maybe Order of the Thorne: TKC?

Chronology

Gateways

Lilly Looking Through

Stories Untold

Puzzle Agent

Puzzle Agent 2

"Renowned Explorers: International Society" can be short, depending on settings.

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https://www.gog.com/game/ohsir_the_insult_simulator

good fun and doesn't take long to complete, and you can do it over and over with different character and unlockable extras.

You can finish any of these games comfortably in less than an hour.
Metal Slug series
Shock Troopers series
King of Fighters
Twinkle Star Sprites
Conga Master

These you can finish in less than 2 hours:
140
Another World
D: The Game
Fossil Echo
Pulstar
Ironclad
Late Shift
Lilly Looking Through
Virginia

I really like this thread. :D Big PLUS to the OP. Being serious here. Every once in a while I crave something short and sweet and the thought of a 40 hour NWN run is too much to even consider.

Thank you

Superhot, took me about 2 hours to complete that one.

Just tried it and it's awesome:
"Your mother farted on a muppet and smells like your cousin's car and I have proof!" Dead.