For the moment the only game I would love to see on GOG is X-Com. It doesn't even have to be all of them if it's at least the first one. I'm dying to try this game but it's hundreds of dollars on Amazon and the only other place I know of is Steam, and I'd really rather not go that route.
I hear the UFO and Jagged Alliance series are note-worthy replacements but I've seen X-Com playthroughs on YouTube and there's just something about it that the other two lack.
If I can look forward to X-Com here in the future I'll be patient; if not, then I guess I'll have to bite the bullet.
Jagged Alliance 2 is for the most part far superior to X-com. A lot of times people will give much greater praise to the older or 'original' game that preceded another game in the same genre(in this case squad-based tactical strategy)for no reason other than nostalgia.
X-Com was a great game for it's time and deserves a place on almost anyone's top 30 list of PC games but Jagged Alliance 2 has to this day been unmatched in terms of what you can do and the ways you can do such. The different dialogs for the various mercenaries alone is worth the price of admission. I have been playing the game for over ten years now and still have not seen %80 of the clever lines included for various actions by various mercs.
Silent Storm is a great 'spiritual successor'(uses an identical RPG/tactical system for the most part) but falls just slightly below JA2 for various reasons.
A big part of what makes X-COM amazing is the base building/research layer, which is much less prevalent in JA. JA's tactical combat is definitely deeper, and both are great games, but if you like the base building/interception/etc. in X-COM, then Jagged Alliance won't have everything you want.
I'd like to beg for Terra Nova here. I'm sure the rights are terribly difficult to secure anymore, but as one of the very few people to have owned and played it back in its day, that game was really damn fun.