It's rather unfair to separate the music from the rest of the programming anyway.

Most composers and musicians of the late 80s were also programmers and had a big impact on the game design. Chris Hülsbeck (on Amiga) and Jochen Hippel (on Atari ST) may be the two most famous, Hippel creating an Amiga emulation on a ST which then was used by Hülsbeck to create the 7 tone music in Turrican 2. Hippel later used that again to create his own soundformat.

If these musicians don't count, then graphic designers should not count as well. Music and sound design are just as important to how a game feels as the graphics, maybe more.

Pyramid Plunge now renamed to Falling Out was done by a single dev studio called PolyCrunch games

The Shivah by Dave Gilbert / Wajet Eye games

DARQ has been developed by a single person ; really impressive game !
Same thing for Titan Station, created only with Blueprint in Unreal Engine.

The music for Dusk was by Andrew Huschult (not sure of spelling) and the multiplayer was by a third party as I recall.

UnReal World was made by two guys, it was first released in 1992 and is still being updated to this day. It even got Guinness World Records for first open-world survival game and for longest update support.

Fun fact, the devs live in the woods just like the characters in the game.

WORLD OF HORROR is developed by a single person, Pawel Kozminski, AKA panstasz

Honorable but questionable mention:

Universal Soldier The game uses the unchanged Turrican 2 code and level design. One guy replaced the graphics (the Turrican armor with Van Damme, the bosses with Dolph Lundgren).

The Momodora series (Edit: plus Minoria) began at the sole hands of Guilherme Melo Martins, AKA rdein. The most recent entries might have been the result of a team, but I'm not even sure of that. It could all be his work.

Ocean's Heart is the work of a single person, Max Mraz.

Treasure Adventure Game was developed by Stephen Orlando.

Not on GOG:

Borington: Rum & Gun (ARPG) and BlackChain (RTS). On Steam.
Chris Sawyer: Transport Tycoon (Closest would be the open source version OpenTTD, though that one is not made by one person).

On GOG:

Eric Barone: StarDew Valley
Eric Chahi: Another World

EDIT: aamok already mentioned Lucas Pope....

What about Grimoire? Wasn't it some guy's pet project?

the problem is that it was about GREAT games....

I enjoeyed Grimoire a lot more than a lot of game made by multiple persons.

Mindustry - by Anuke

Shapez - by Tobias Springer

Nightmare Of Decay by Checkmaty
Dead Beacon by SelianiDEV

Both are really amazing games made by a single person. If you're a fan of classic survival horror games, you should definitely check them out.