Hey GOG Community!
I'm a brand new member of this site, and I'd love some advice from some of the old hands here. I'm looking to purchase my first Good Old Game, and I'm really unsure about which one to get.
I tend to like most every genre, but I really can be a bit picky when it comes to particular games. I really want something with deep gameplay, contemplative pacing, and an expansive scope. I'm huge fan of games like Civilization, the Elder Scrolls, and the original Operation Flashpoint and Delta Force games.
So, does anyone have any advice?

Heroes of Might and Magic 2/3 is really good if you haven't played that. I think you'll also find yourself right at home with the Jagged Alliance series - they were the first games I bought. Lords of the Realm is also insanely addictive.
Games with larger playing fields include Sacrifice, Giants, Hostile Waters, Might and Magic, and Interstate 76 - these IMHO are some of the best that GOG have to offer, although admittedly Interstate 76 is rather buggy. These games are all equally good though, if you're looking for an expansive game world.
And of course, there is Gothic and Two Worlds - these are the closest to The Elder Scrolls series that you'll find on GOG.
I would recommend Messiah but it is more of a love-hate game - it's supposedly more non-linear than the typical third person shooter, but it's controls are quirky and it is marred by gameplay bugs.

I'd say try Jagged Alliance 2. Don't know anyone that was disappointed by it.
And it also fulfulls the deep gameplay part nicley.
edit: Oh yes, can't belive I forgot the above stated Heroes of Might and Magic 2/3.

Master of Orion 1+2 cause you like Civiliztion.

I will add Divine Divinity, Sacred Gold and Two Worlds . . . =)

/facepalm I forgot GOG has MOO2. Yes, get MOO2 if you have never played it yet. It has lowyhong's Rainbow-Gold Seal of Approval.

hmm... you might like the Fallouts (1 or 2). They're isometric top down RPGs in a barren wasteland. But MOO 2 is good also, wasted a lot of time when I was younger on it.

Master of Orion 1 and 2

The Fallouts would be a good choice. Also Tropico Reloaded if you're looking for something managerial (although I generally recommend Tropico 3 over it). Also. Outcast. I'm surprised nobody else mentioned it before, to be honest. Seconding the Hostile Waters recommendation.

Fallouts without a doubt.

JA2 is a good choice (don't forget the unofficial patch:
http://www.gog.com/en/forum/jagged_alliance_series/pre_compiled_ja2_stracciatella_unofficial_patch_for_windows)
Fallout 2 is also a good one (Killap's patch highly recommended:
http://www.nma-fallout.com/forum/viewtopic.php?t=54328)
Master of Orion 1 + 2 - an essential strategy pack, yes.
If you want something Civ-like - Call to Power is not that bad, provided you applie the community patch - really mediocre without it. (http://www.gog.com/en/forum/call_to_power_2/apolyton_ctp2_edition_revision_1097_07_jun_2010)
If you want something less... convenient, you might try Giants: CK or Sacrifice.

Since no one has yet mentioned it, I'll throw in Empire Earth Gold (or its first sequel).
Why?
Deep gameplay - Well, not that deep, the build-up is a quite generic RTS setting, but you can toy around a LOT with custom-made civilizations.
Contemplative pace - CHECK. Even though it's an RTS game, the game plays out nice and slow.
Expansive scope - CHECK. The game can be played anywhere from the Stone Age to the distant future, spanning 15 epochs. Also, you can generate some very large maps, plus the game features some nice (albeit generic), (mostly) history-inspired campaings.

Bad Elwin!
/Fenixp slaps Elwin over his hand
You shall recommend Fallout 1 first, for chronological order and atmospheric superiority of the first part are important!

Heroes Of Might And Magic 3 - Still addictive and a huge amount of content and then there are the mods.
Jagged Alliance 2 - My desert island game without a doubt, plus with the 1.13 mod it just gets better.
Hostile Waters - Excellent action/strategy game that is fast paced and pure fun and a game i didn't even know about until GOG kindly showed it too me.
Gothic 2 - Massive and very challenging rpg that requires effort and patience but is more than worth it.
Master Of Orion 2 - Waited years to play it, thought it was a solid case of rose tinted glasses when i took my first glimpse at the starmap and played a few turns...still playing it and still lovin' it.
Realms Of Arkania 1&2 - Old School and impentrable at first but all those numbers and skills really start to take shape as your very own party of adventurers, and the adventure is certainly one worth playing.

I would recommend Fallout 1+2, Sacrifice and Giants: Citizen Kabuto.