The difference is the Gammelost isn't actually a thing in Norway. I grew up there and never saw it. Whereas Brunost is staple food eaten every day by a large part of the population.

There's a list of foods that are, I think, used to scare foreigners with for our amusement more than actually eaten. Of course some crazy Norwegians do actually like these, which gives us plausible deniability when said tourists and foreign co-workers question whether it's real. I'm sure you are fully aware, but for those unaware of Norwegian cuisine:

* Smalahove (whole, barbecued sheeps head; how whole depends on who/where, but the eyes and brain certainly need to be included)

* Lutefisk (fish half-dissolved in caustic soda, then washed out - or your intestines would have a really hard time - leaving what is best described as fish-yello)

* Gammelost, of course.

Then there are the less objectionable or outright nice things that we still serve to foreigners either knowing they're acquired tastes, or that we like spinning stories around to try to make it uncomfortable for our entertainment:

* Whale, presented as "Willy from Free Willy". Whale tastes fine - it's just a bit tough, and sometimes a bit oily and "fishy" (yes, I know, not fish; doesn't stop the blubber from affecting the taste)

* Deer and reindeer, which tastes great but squeamish people everywhere get more squeamish when the dish is introduced as "Bambi". When I worked for a US company in Norway many years ago, the US CEO came for our Christmas party and the CEO of the Norwegian subsidiary had of course ensured that the menu was moose, deer, and whale, so that our entertainment was to watch the CEO's reactions (he took it well).

* Sour cream porridge. I love sour cream porridge, but it is an... acquired taste, and people do not expect it if you put a bowl of porridge in front of them without warning...

* Salty licorice full of ammonium chloride. Yes, licorice with floor cleaner. I love it, but outside of the Nordic countries and the Netherlands, the only person I've made taste this who has liked it was a single co-worker about a quarter of a century ago. I keep bringing them to the UK at least twice a year and imposing them on unsuspecting people (in the UK, finding even lightly salted licorice is near impossible outside of specialty imports, so people here are caught entirely unaware of what salted licorice will taste like, and that is half the fun). For science, of course.

Norwegian humour when it comes to feeding people stuff they're not familiar with is pretty basic, along the line of "we're serving you a pet/beloved children character" or "we're serving you something likely to dissolve your intestines/an industrial byproduct that our children eat". Where my Nigerian in-laws worry about whether the food they're serving me is too spicy and fuss over it constantly, we try to trick people into trying weird things while we take pleasure in observing their reactions and/or try to stifle laughter.

Eh, large part of the population is relative. I'd guess 1/20th, or something like that.

Im Norwegian, and I have never been in a house where anyone had Gammelost. There are people who eat it of course, but it's mostly a thing we scare foreigners with.

Gammelost is just fermented cheese. No biggie.

You've had it, or you are going by the written description?

I only tried it once, but it's the only cheese I've been unable to eat. Sawdust soaked in cat piss would be comparable but preferable. I was exited to buy it and brought it along for a hiking lunch. My wife and I both took one bite, and then decided to bury it under a rock. I felt sorry for the rock.

I still do wonder whether I somehow got a bad batch, or one that was spoiled.

I've had it. I'm Norwegian, so my father bought some on a dare at one point.