For me I not sure why you think it does not work 100% as intended and advertised to you at the place where you bought it.

Imagine a product produced for two separate shops, same product with slightly different features for each shop. You couldn't buy it from shop A and then claim it was falsely advertised/not as described because you went into shop B and it was advertised with having different features there. It would be good if they did refund, but you probably realise they wouldn't have to.

The creators have stated it is cross platform in terms of Win/Mac/Linux only not between anything else - it is the very top result in the first search I did, where they directly respond to someone on reddit and say this (the top reddit post in this top result).

I am glad you got a refund, I genuinely think it is good that they decided to do it as it shows they can make decisions and not just stick to a rigid policy. But I think you should accept it is in spite of you not being entitled to one, not because you should have been given one.

Unfortunately in this case, cross platform multiplayer might mean you can play against people on Steam and PlayStation (based on what I see on Armello's website).

GOG's page doesn't mention anything about playing against people on Steam and whatnot (that I saw at least). Probably wouldn't hurt to post on Armello forums to see if or when that might become a reality.

While I understand yout frustration with this, I can see fully why GOG wouldn't refund you, as despite you can't play the multiplayer with Steam friends, the game does otherwise work in your machine, so there's nothing that would really make you keep your part of the bargain and delete the game forever if GOG would refund you. As there's no DRM, your copy, if you keep it, is always valid.

I wouldn't expect a refund in this situation, being able to play multiplayer against Steam users isn't an advertised feature, so that doesn't really make it qualify for the "game not working" refund. It's nice of GOG to do a store credit refund anyway though.

What I find ridiculous is that the developer supports multiplayer in the DRM-free version but didn't include an option for that mode in the Steam version.
While it's stupid of the devs not to allow the Steam version to play with the DRM-free version, it's not something to be expected unless advertised. Your definition of 'not working' is not one I'd agree with. It's a disappointing omission, but not what I would consider broken.

As I said, I don't think it's GOG's fault at all. It's the developer's fault. And I think that it's reasonable to expect that a game advertising multiplier would have said multiplier work between games on the same platform, particularly if they are advertising cross-platform multiplayer (it's like... you made it work crossplatform... but didn't make it work on the SAME platform?).

Personally, this will make me leery of purchasing multiplayer games from GOG again (and that's most of what I buy). That isn't meant to be a threat or anything, it's just precautionary on my part :D