Maybe it's the other way round: you keep ticking "other", and if that triggers something somewhere, they will eventually get back to you and ask what this "other" might be. I don't think they'll outright ban your refunds without clarification, but I do see your concerns, as this generous refunding policy indeed is to be used with care in order to keep it available to all of us.
Still, implementing a conditional dialog box and evaluation / safety checking (must always consider hacking attempts using all sorts of misformatted input) might be too much trouble when most people just tick "didn't like it", anyway.

I've never refunded a game, so wouldn't know, but I guess it is all about their records and not really about the specifics of 'Other'.

So no doubt what they consider are important reasons to pass onto the game provider, is what it is mostly about, and something like motion sickness is a well known thing and fairly common for some folk, so doesn't need a spotlight.

To give specifics for 'Other' would require GOG caring about the specifics, which I don't think would occur unless you are refunding too often. If that happens then they may well contact you, but unless they recognize something from a doctor and accept it, GOG would have no idea how truthful you are, and it is a bit like a well known factor so not exceptional enough for an excuse ... in other words you know you have such an issue and so should be choosing certain games only etc.

Not particularly strained position of the neck, although I wouldn't discount that completely because blood-pressure to the brain and fluid pressure in the inner ear may indeed affect a person depending on the disorder.

It also may have something to do with the crystals which can build in the labyrinth with some people - constant movement of the head can help move such buildups to benign areas or help break up said accumulation. So frequent slight changes to head position over long periods may indeed help those people.

But on average it's more of an accumulative exposure to stimuli which the brain cannot interpret in synchronisation. The eyes see one thing, the balance centre feels another, the brain doesn't feel it's correct, and the more and more you're exposed to this mismatch the "sicker" you feel. It eventually can trigger lasting headaches, nausea and yes, nystagmus & resulting vertigo. All of these can lead to vomiting and the need to lie down until it passes. Nasty.

It can occur in different parts of a same game. For instance when playing The Long Dark I wasn't being affected much at all for hours of constant gameplay, until my character was caught outside in pitch blackness with a scrolling snow-effect in vision across the screen. With no input of movement other than slight jolting of the steps jarring the snow, and a constant fake snow effect on the screen, it triggered the nastiest full-blown vertigo, headache & eyestrain event I've ever experienced. Took half and hour for me to get back to normal. Had I had a torch or flare in the game at the time, likely I would have been fine, because I would have had more reference for the movement of the character. Head position had nothing to do with this particular situation, but yes I have played other games and found my head tilted to an unnatural angle which may have exacerbated things. :)

I've never refunded a game, but reading through the responses here, I can't help but think that GOG should have a box where you can type in your reasoning for refund, and ditch the check boxes altogether. If GOG wants to classify the responses, let them do it themselves.

That additional box might make you feel better, but likely GOG would just ignore it, so it would just be an illusion and nothing of substance.