There are different kinds of tests, for different purposes.

I presume GOG repackages the updated files they have received from the developers into a GOG patch installer, and sanity checks that the patch installs correctly, and doesn't appear to break the game installation. Not all developers use the same naming scheme and installers for their games and patches, so someone has to change those to "GOG format" and naming scheme anyway.

You can see the opposite in e.g. HumbleBundle where they allow the developers to use whatever format they want and HB doesn't meddle with it => the HB store is quite messy for its various kinds of DRM-free installers. GOG.com wants to be more organized with its releases, and I for one commend them for that.

I don't think GOG really play-tests the patches, ie. confirming if it really fixes all the generic gameplay issues that it is supposed to fix, and doesn't introduce new problems. I think that kind of functional testing is indeed in the hands of the developer, and GOG assumes the publisher has done their testing properly already. Apparently sometimes they don't, and in the end it is the customers who play-test the patches.

Some have suggested that if GOG required the developers to do the formatting and sanity checking for the GOG patch, and offered a more automatic way to release patches on GOG (e.g. through Galaxy), then such delays wouldn't happen. I am not quite as convinced, most publishers would most probably still prioritize the Steam patches first, and publish the GOG patches (and any other versions) only after they are done with the Steam patch.

What can GOG.com do in a situation like that? Sue the developers for prioritizing Steam over them? That still wouldn't speed up the release of the GOG patch. At least when GOG decides to prepare and sanity check the GOG patch themselves, they are taking matters in their own hands, instead of waiting when (if ever) the developers finally release a fully tested and sanity checked GOG patch.

Ah. I was unaware of this and assumed people claiming that the delay was GOG testing patches revolved around making sure the patches didn't introduce new problems. That's my mistake, then.

Maybe have some kind of opt-in for patches that doesn't require an installer? Just the raw files to overwrite? I'm not even sure if that's possible, and I'll freely admit to being totally out of my league in this conversation, but anything that puts more control into the hands of the customer would definitely be a step in the right direction at least in terms of how the patching situation looks.

It'd be nice if they worked in some kind of patch "parity" arrangement when negotiating new games so that there's something that devs/publishers are bound by, too. Again, I don't know if that's really possible, but it too would be a good step, looking at this from the outside.

Here is a solution that will provide patches as fast as developers releases them:
gog.com /wishlist/site/a_vote_for_voting_on_useradded_external_links_on_product_pages

So you vote for links that users adds.

A section for reporting issues with games would be nice as well. Like WineHQ -- but if you run the game with Wine, you should use WineHQ of course, as long as the patch GOG uses matches the one on WineHQ.