With the modding scene apparently bound to Steam Workshop now, I'm just wondering if there is or ever will be a way to add the Slimebound etc. to the GOG version of the game?

I was wondering too if mods were possible to install in GOG's version before buying, so I second your question.
And is Galaxy required to do so ?
And is it possible to install steam workshop's mods ?

There are still working mods on GitHub, but this might eventually break because apparently active development has moved to Steam Workshop:

https://github.com/kiooeht/ModTheSpire/wiki/List-of-Known-Mods

A friend has tested this successfully with a few mod characters (e.g. Slimebound), so it still seems to work.

So it sounds like no real mod support in the long term for GOG users for now, unless devs do something (or if each modder thinks about us) :/ Because if the game is updated, old versions of mods on this page risk to not work anymore. If they were still available.
Steam workshop is really an issue...(this is not the first time this kind of things happens) but well, thank you very for the information !
But let's be optimistic, the game just released recently, so hopefully they'll hear us and do something :)

we should tweet at the devs - MegaCrit has been pretty responsive on Twitter from what I saw during Early Access...

That's a good idea ! But I don't have a twitter account so... could you do this if you have one ?

On my side, I have send them a message on their Discord (in the feedback channel), explaining that GOG users can't download mods when they're on steam workshop, and asking if they were planing to do something to help us. It has been automatically saved, but it may take some time for them to see it :P

But it's nice to hear that they're responsive :)