So far as i know, GoG-Games are sometimes lacking a new upgrade, not necessarely because of the devs but because GoG is simply to slow updating their launcher.
I think GoG needs more "power" but seems to lack the ressources doing so. Devs are very rarely, if ever, interested into publishing "outdated" versions, because it will only cause unnecessary issues. In extreme cases someone will simply get a "updated crack"... issue solved.
As for missing content such as sountracks or artbooks, it surely can be downloaded somewhere, as soon as Steam content is cracked. Bad treatment of paying customers is always a worst-.case solution and it will NEVER be any good idea. But of course, no good feeling, knowing you was paying for it and finally... need to get something "out of a legal instance"... sad.
Sometimes even many DLCs are missing, but i guess there will be mods available for a even better upgrade. Usually those kind of games i put on hide and saving me the nerves, just not worth it. Maybe you can put some bad words to the devs and/or GoG, in case they did not update and files already available to them. I would usually not go further than this... and usually the games i get are complete.
Indeed, some devs/publisher simply got in mind "GoG is a worse second class citizen platform, we will give them some cheap cutted down-version,, as their genital-size and moneybag is low sized already"... however, this is... fortunately, not the majority.
Surely 2 currently worse offenders:
WARHAMMER. CHAOSBANE: Several DLCs missing including 4k textures (weird enough, GoG is NOT removing the line "THE ULTIMATE VERSION OF THE GAME: unlock all DLC with this edition to enjoy an enhanced game" experience and unlimited replayability!) https://www.gog.com/game/warhammer_chaosbane
I was putting the game on hide now, just not worth the troubles.
And the second worse offender:
NECROMUNDA: HIRED GUN:
Every DLC missing, and apparently the devs do not care. Steam is basically "premium" to them... seems to be the reason. https://www.gog.com/game/necromunda_hired_gun
It does not affect my gameplay in a negative way, and the missing content i may get somewhere else... in term never updated.
Of course there is many more offenders....