Serious Sam's Bogus Detour took me just under 6 hours according to Galaxy. There's also an achievement for finishing it in less than an hour, good luck with that one. :o

Alien Rampage took me just about 4 hours to finish it.

And since they're here now and weren't on the list before, the following can be completed comfortably under an hour:
Metal Slug
Metal Slug 2
Metal Slug X
Metal Slug 3
Blazing Star

Not up to 5 hours but:

Return to Castle Wolfenstein - I can finish this game in about 7 hours.

Hellblade: Senua's Sacrifice is pretty close to a one-day game. According to Galaxy, I've spent 7 hours 4 minutes in it, and in that time I've finished the game with 100% achievements, spent a good amount of time needlessly backtracking (the atmosphere is fantastic), watched the Hellblade feature video, and had left the game idling while doing other chores. I'm certain the game itself was just about 6 hours.

Skylar and Plux: Adventure on Clover Island takes 2-4 hours to play through.

Dink Smallwood's base campaign is quite short IIRC.

Pinstripe takes about 4 hours to play through.

Silence could be finished in one day, too, if you have a little more time to spare. I think it took me ~6-7 hours, and others might finish it quicker still.

Wonder Boy: The Dragon's Trap would also qualify.

Mousecraft

While the game has about 80 levels, they aren't particularly difficult. The game can be beaten in around 3-5 hours

Super Chibi Knight (not on GOG) takes 2-3 hours to play through for one specialization path (beast master or sorcerer). If you want to see both, I guess that would be another 2 hours or so.

Not sure if that still counts as "one day", and neither is available on GOG, but both of these don't take more than 6 hours to play through (and probably less on average):

Valley
Hero of the Kingdom II

GOG:
The Darkside Detective
Gorogoa

Dear Esther should be moved to the GOG list.

Non-GOG:
The Count Lucanor (about 5 hours to win with a decent ending, 10 to exhaust all imaginable replayability)
Sorcery! parts 1 and 2, part 3, part 4 (very replayable but easy to meaningfully win: undo action, no dead ends, no achievements either in the game or on steam)

Actually, when you progressed further into the the game and unlocked the reusable block, the game got considerably easier. There weren't that many levels that stumped you but I can't remember exactly how many, I remember at least 2 but I'm sure there was more than that. Figuring a solution to collect all the shards and mice wasn't particularly tough either and I never needed to use a walk through.

I should probably clarify that while I did finish the game in one day, I did it in two sittings. It was a crappy day and didn't have much going on so I finished a good portion of the game in the morning, did something else in the afternoon then finished it in the evening.

Cayne can be finished in 5 hours.

Just finished Late Shift in ca. 90 minutes. But the fun in the game seems to be to try different choices and see how the story changes so even though I consider the game finished there's still something to see.