The 5 games (or series) I'd most like to see on GOG, in no particular order:
Fate: Gates of Dawn [url=http://en.wikipedia.org/wiki/Fate:_Gates_of_Dawn]http://en.wikipedia.org/wiki/Fate:_Gates_of_Dawn[/url]
Released for the Commodore Amiga and the Atari ST by Reline Software in 1991, this is a game where you play a lone adventurer who accumulates various party members, and eventually up to 4 separate simultaneous parties, in the course of his adventures. It reminds me a bit of Wizardry 7/Gold now, though it precedes them, but its combat- and exploration-laden style along with the manual cartography on graph paper that the player would find indispensable combined to make this a game in which an investment of over a hundred hours wasn't unusual at all.
Ultima I-IX, Ultima Underworld 1&2, Ultima: The Savage Empire and Ultima: Worlds of Adventure 2: Martian Dreams
http://en.wikipedia.org/wiki/Ultima_series Spanning nearly 20 years, this multiplatform (I started with Ultima III on my Commodore 64) collection (created by Richard Garriot and published by everyone from himself (and sold in plastic zip-lock bags) to Origin to Electronic Arts) of fantasy role playing computer games implemented just about every feature possible for its various periods, from single character play to a full party of 8 characters, from turn-based to real-time combat, from amoral play to games where stealing and murdering led to unavoidable and negative consequences. The games have been ported (and drastically changed in the process) to consoles ranging from the NES to the Playstation and to the original Game Boy handheld system. Along with the Might & Magic and Wizardry series, the Ultima games are an essential component of every hardcore computer role playing game's resume; without at least a few Ultima notches on your virtual fuselage, you simply can't call yourself a veteran CRPGer.
Wizardry series
http://en.wikipedia.org/wiki/Wizardry Developed and published by Sir-Tech (see the Jagged Alliance games carried by GOG for a much different example of the company's work), this series lasted 20 years, from 1981 through 2001, and appeared on platforms ranging from the Apple II to the IBM-compatible PC (Windows 98 and XP). The graphics on the initial several games were quite primitive, with just a few vector lines, bulky pixels, and loads of the user's imagination required to compensate for the technical deficiency, while in the last installment were they fully 3D accelerated (and still hold up to today's standards if you ask for my totally biased opinion). Turn-based combat remains consistent throughout the series, as did the option to import characters from one game to the next, which in later installments meant a different beginning than that for newly created characters (multiple endings were also standard fare).
World of Aden: Thunderscape [url=http://en.wikipedia.org/wiki/World_of_Aden:_Thunderscape]http://en.wikipedia.org/wiki/World_of_Aden:_Thunderscape[/url]
Published by SSI in 1995 (I guess SSI had lost the rights to continue developing games based on the AD&D system), this game played a lot like the later Might & Magic 7 in that the combat could be done in real-time or in a turn-based manner. It also featured a party of 4, a 3-D view above the character face portraits (regular VGA and not 3D accelerated if memory serves). A commendable effort to carry on making CRPGs after the AD&D franchise was lost, and though it wasn't very successful commercially (though it did spawn a sequel that used a completely different (isometric and real-time-only) engine), I for one would be very pleasantly surprised to find it on GOG someday, though I'm not holding my breath.
Dark Sun: Shattered Lands and Dark Sun: Wake of the Ravager [url=http://en.wikipedia.org/wiki/Dark_Sun:_Shattered_Lands]http://en.wikipedia.org/wiki/Dark_Sun:_Shattered_Lands[/url]
Developed and published by SSI in 1994 and 1996, these turn-based games were based on the AD&D engine but looked quite different from the gold-box AD&D games that SSI had made prior (not to mention from the real-time first-person perspective Eye of the Beholder titles with which it was also involved). A bit more graphics flash, and improved audio, including CD-quality Redbook format heavy metal music on Wake of the Ravager. A worthy successor to the classic gold-box games on which SSI had cut its AD&D computer adapting teeth.
I've got many other suggestions (Knights of Legend, the remaining Might & Magic games, Heroes of Might & Magic IV&V, Celtic Legends, Drug Wars, etc.), but I haven't the time to elaborate on those without missing essential rest.
