Korell: There's a problem with this, though. Some of the GOG extras aren't really extras. For instance, manuals and other reference materials that came with the game. Not only are these not available for purchase separately, but they shouldn't need an additional purchase anyway, yet as "Extras" they don't get included in the Amazon Prime versions.
As for optional DLCs, do the Amazon Prime versions allow you to buy the DLCs for them, or do you have to get the GOG base game and get the DLCs for that instead?
Amazon Prime includes games for various platforms (GOG, Epic Games, Amazon Games, Legacy Games).
You get a key for every GOG game you claim there, but these are the exact same versions that are available here on GOG's web store. There are no extras omitted or anything. *IF* a product on GOG includes extras, you'll also get them via redeeming Amazon Prime codes. The truth is, that many modern games simply don't have any complimentary extras (manuals, reference cards) included anymore (unlike the old "classics").
Again, there are no distinct "Prime versions" ** of games. If you are looking for DLC content, you don't need to buy another "GOG base game" because they are the exact same. If DLCs are available, great! If not, then it's not a limitation by Amazon.
** Technically, there is a small difference. Amazon Prime giveaways are implemented as special "bundles" in GOG's database. But that difference shows only in the very first step (when you redeem your key). --- Basically, it works like this: Prime gives you games in a fancy gift wrap with Amazon logo on it. Once you open your present (= redeem the key), out comes a plain old GOG product which gets added to your GOG library the same way as if bought directly here in the store.