How does GOG go about collecting the bonus content for games? This is for games already been released as well as new released games. Thank you for the help in explanation!

They make the wallpapers and the avatars.
The soundtrack is obvious.

Usually the rights to offer supplemental material like artwork and soundtracks are included with the rights for the games themselves, so I imagine the rights-holders (EA, Night Dive, etc) supply GOG with the bonus stuff. And many older games on GOG used copy protection that requires the manual, so that's often necessary to be included.

So if we have stuff that could be used as bonus goodies we should contact gog? I have a few better quality videos that are on gog and a few reference cards that aren't on gog

Thanks for the help!

Last time I contacted support about such a thing the answer I got was that in most cases the stuff isn't included for legal reasons, not because you guys don't have access to it, and that I shouldn't contact you guys about stuff like that. So I presume that if I come upon something suitable for bonus material (and not created by third party like say a magazine or just fans) I should still try contacting support about it?

Do you know what happened with the Thunderscape strategy guide? I remember reading a while back that the Kyoudai guys had a copy and were scanning it or whatever, but nothing has come of it since.