http://www.gog.com/games##devpub=activision

It seems there are only a few publishers avaible for filtering the games list?
Will this be fixed?

No, Its the new marketing campaign that they have been talking about on twitter.

Honestly though I dont know, think i only ever used the publisher list once about 4 years ago.

The last few days the filter options haven't seemed to have worked at all for me

I've found that clicking links or dropdown for filtering doesn't work, but typing the search term in the box on the "browse games" page will still filter accurately (for any term, whether it's a title, dev/pub, or tag like "fpp")

I see 412 developers and publishers in the filtering menu, though it seems the filtering isn't always working (may need a refresh).

The filtering is a mess. Don't be surprised if it doesn't work properly. I personally never knew that the DND games were a massive collaboration of developers/publishers, but that's what GOG tells me! :D

P.S.: On a side note, I found it quite amusing when Ultima IV was temporarily the #1 best-seller.

Oh ok :D Well, in my filtering list there are not so many publishers and it also does not work in my account for filtering my owned games =(