It used to be that when you bought a game on GOG, especially an old game, that it would come with a few sundry extras. This often included things like a scanned copy of the original manual, or stories that tie in with the game, like I Have No Mouth and I Must Scream and Planescape: Torment.

Lately though I haven's seen any extras with games.

It depends on the game and the publisher. Some of my latest purchases have extra content, some don't.

Also players used to send in their own scanned copies to share them with others. That may not happen anymore.

Even if GoG has all the extras (provided by fellow GoGlodytes or by themselves) ready to be put up alongside the corresponding game - they still have to consult and negotiate with the rights holders.
And some of which may not want (for various reasons) the extras to be included with the game, at no additional cost.

Same with the different localised versions of a title. The rights to those all lie with different people or parties (most of the time the studios that did the localisations) and each has to be approached and asked for permission individually.

There's also the sad reality that most modern games don't even come with any extras. And if - then only in ''deluxe supporter ultimate edition'' that costs €€ extra.

But even things such as manuals!? Manuals are a lost art.
Forget about any ''making of'' videos or concept artbooks.