Well out of my head I can count EA, Lucasarts and Take-Two as major players that are missing from GOG. For first two, I can count at least extreme paranoia over their IP as reason and wanting DRM to protect it as reason not to be at GOG. EA is also perculiar that they tend to sqeeze their IP's dry then hoard them in huge pile and protect them like angry red dragon no matter what is offered for them.
Obviously there are many other middle and small time developers and publishers out there. For example Paradox Interactive. Don't know why they're not at GOG as every other DD store sells their products, they already sell them DRM free at GamersGate and many of their games would fit GOG catalog both price and age wise.
Here are few problems that may put of devs/publishers from selling their games at GOG:
Games must be allowed to be sold DRM free
Games must be allowed to be World Wide without restrictions
Games must be allowed to be sold for 5.99$ or 9.99$ for everyone everywhere (no 1$=1€ currency scam, no regional pricing, preset price points by GOG).
Games must be allowed to be modified by GOG staff as needed to ensure compatibility with modern Operating systems
Other problems GOG has:
Actually finding who owns the rights to each game and actually locating them (not easy when devs/publishers have gone bust or rights sold several times over)
Locating usable game disks or files when rights owner(s) don't have any stored and stripping any DRM/copy protection from them (in many cases the pre-print DRMless Gold Master disks/files and source code are either lost or unusable)
Making games work on modern Operating Systems (not an easy task in some cases, almost impossible on others)