You're wrong about GOG not being in involved in the creation of the patches they distribute. Game developers supply the contents
for patches, but distributors like GOG and Steam create the actual archives/installers from those contents, and they put them through their own QA/testing pipeline. When there are problems with patch installation (and there have been), then that's absolutely GOG's fault, not the developer's.
And it's also GOG's fault that the patches have a much larger download size than necessary - look at the unofficial Linux patches for Pillars created by a user on these forums, they're a fraction of the download size and deliver exactly the same content.
It's also GOG's fault that they don't combine patches to make them cumulative. For going from 2.00 to 2.03, you would need to install three patches in a row: 2.00->2.01, then 2.01->2.02 then 2.02->2.03. It doesn't have to be like this. Cumulative patches are not rocket science.
It's also GOG's fault that they remove previous patches from the download page much too quickly - like when the 2.00->2.01 and 2.01->2.02 patches were removed when 2.03 came out. Why didn't they leave them up for at least a few more weeks? Why does the download page not have a "show older patches" button?
You can't blame the game developers for things like that.
Fact is, GOG's patch delivery for new (a.k.a. frequently patched) games has been a mess so far. When GOG sold mostly old games (which didn't get new patches anymore), those issues did not arise. But now that GOG distributes more and more new games like Pillars and Witcher 3, they really need to improve in this department.
First of all, I think we should establish what a patch is--ie, the "contents" of the patch and "the patch" are indistinguishable to my way of thinking (If you don't have the "contents" you don't have anything)...;) As well, I have read that a dev with a Steam game and account has access through Steam to patch his game directly...they upload directly to their developer's Steam account and the Steamworks software patching mechanisms apply the patch. I've read quotes from both Obsidian and inXile that may be paraphrased as "We uploaded our patches through Steam today, so those are immediately available, and we know that GOG will follow suit in a day or two." There would have been no time for Steam Q&A (of course.)
It doesn't really make sense that Steam would be Q&A'ing all of these patches--as that is properly the developer's job and I'm quite sure the developers must do it. IF GOG (or Steam) were the ones doing the patching, not to mention the Q&A, then that would imply that GOG/Steam have the source code for those games and free reign to do anything they wanted with each game they patch. I don't think so.
Valve doesn't even Q&A some of the terrible game releases(!) they've offered through Steam--which would have saved them considerable trouble with refunds and bad press. My opinion is that GOG tells the developers what it wants and how it wants it when it comes to patches--but that GOG has little influence over what the developer's give them
--if the developers decide they would rather do something other
than follow GOG's guidelines & suggestions. With Steam, however, the developers control everything, including the DRM involved (or its absence).
I think it was with a D:OS patch I noted that my GOG patch was quite a bit smaller than the Steam patch, by gigabytes(!)--but the Steam patch, as usual, hit a couple of days earlier. I didn't say that GOG wasn't "involved" because obviously GOG has to take what its given them by the developers & inject it into their own file distribution system--the developer can't do that any more than GOG can create the game patches...;)
The thing about cumulative patches is that they appear chronologically in line with the developer supplying them to GOG. So, the reason GOG doesn't immediately combine all of the cumulative patches together into one big patch is because when 2.01 was released it was weeks/months before 2.02 was released, and weeks/months before 2.03 was released. So what GOG does when it can is the sensible thing: after 2.03 is released they keep 2.01 and 2.02 & 2.03 available for people who are working from the 2.00 version, but they then make the initial download file version 2.03 for the whole game--so that people just buying the game will not have to incrementally patch at all. I've seen nume3rous instances of GOG doing exactly that. GOG gets faulted for doing both by some people.
If you believe that GOG deliberately creates patches much larger than necessary (which I still have to say I have not observed), why do you think they do so? What's in it for GOG?