Dungeon Master 1+2

Max Payne. Like seriously wtf. The original Max Payne defines the term "good old game."

Ditto for Total War. The original Shogun and Medieval at the very least should be here.

Also Assassin's Creed II, Brotherhood & Revelations. The Ezio trilogy is iconic and more than 10 years old. It belongs on GOG.

The original version of Z, instead of the remake that's sold on GOG - and that is not so hot, judging by the reviews... really, why not offer the DOS original as a bonus, at the very least?
Also, Gruntz. But maybe, as for Gunlok and Get Medieval, the reason is that it's too obscure and wouldn't sell that well.