Great, so my mind IS going.

Return to Zork was one of the hardes "available games on GOG" games I've played....simply because the mouse input & movement scheme in RoZ was so clunky & super laggy.

Normal movement between game locations in RoZ was bad enough, because it relied on the mouse controls & clicking at weird angles on the edges of every location.
The mine section in Return to Zork where you had to go left/right/forward each time a new intersection comes up was unplayable & unwinnable because it was pure QTE (quick time events) with super laggy mouse input.

Stalker: Clear Sky is pretty hard.

you can purchase a ton of buggy, unfixed, outdated and heavily broken games here on gog and they're all really, really, REALLY hard to play!!!!! try rage of mages 2 for example.

but somehow i don't think that it was gogs intention to make their titles a hard experience, its just release pressure by gogs greedy shareholders.

Try Volgarr the Viking. it is great.
Also, Blackhole.

It it's one of the most crash-prone games I have and I gave up on it. SoC I finished without problems though.

I always feel like 2/3 of the difficulty of Volgarr stems from not having air control when jumping.

HoMM3 is bonkers. I frequently feel I do alright till I encounter the enemy and he has 4-8 times my number of units.

La Maluna has doable combat, but it's tough as nails to figure out what to do next (especially if you overlooked something minor).

making "realistic" jumps was a precise intention of the devs. Like in older Castlevanias and in real life jumps, you cannot change your trajectory mid-air. The only way to do so in Volgarr is double jumping.

Also... imo, La Mulana is the unfairest game I have ever played. Too cryptic for its own good, small details everywhere you have to write down and figure out when to use. I didn't really like the game, but it could be the best ever made for a superhardcore explorer.

This one.

Kind of repetitive but the puzzles vary slightly with each iteration.

The Meat Circus in Psychnauts before they patched it. I just gave up and youtubed the ending. The patched one is about right.

Also, Kights and Merchants is hard just because it's horribly designed. It's not fun.

I heard We are Dwarves is quite hard. I haven't yet tried it.

I gave up on the patched version (I think). What did the patch change?

"Dying is easy, comedy is hard."

I didn't really have any problem with crashes with it though, ran almost perfectly. It's just a very hard game, especially in the beginning.