I have 104 games at the moment on my GOG shelf.

Here is my list. I have only included one game per series:
Gothic: I don't have Gothic 2 on GOG, otherwise it would probably replace the first one.
Earthworm Jim 1+2: one of the first games I got to know, played the crap out of the demo. After Gothic, this was my second purchase on GOG.
Gorky 17: I know it's generally not considered to a be a *great* game, but I can't stay away from it. I love the combat system, the graphics, the atmoshpere, and it's just one of the few games that I am happy to play anytime.
One Unit Whole Blood: my favorite of the Build engine games.
Stronghold: memories of a wonderul, warm summer with soothing breezes, sandy beaches... that was spent indoors in a poorly lit room because of this game.
Jagged Allliance: one of the best games ever. If I could ever make a ''top few games'' list, JA would be a set-in-stone entry.
Fallout: the first RPG that I truly got into. It's just a great game.
Dungeon Keeper 2: a really cool premise and a very enjoyable game.
Disciples II Gold: I like it better than Heroes...
The Incredible Machine Megapack: funfunfun.
Sanitarium: Day of the Tentacle is not on GOG, and I just have to have a point'n'click game on this list. It's an excellent game in its own right, though.
Stonekeep: a truly underrated game in my opinion, one that I've just recently [re]discovered on GOG.

12 out of 104, roughly 11%.

Ok, I got 162 games...

My essentials so far are:

Baldur's Gate
Baldur's Gate 2
Blood
Broken Sword - Shadow of the Templars
Deus Ex
Duke Nukem 3D
Fallout
Fallout 2
Freespace 2
Gabriel Knight
Heroes of Might and Magic 3
Icewind Dale
Icewind Dale 2
Neverwinter Nights
Planescape Torment
Thief Gold
Thief 2 - The Metal Age
Thief - Deadly Shadows
Unreal Gold
Wing Commander 1+2
Wing Commander 3
Wing Commander 4
Wing Commander Privateer

23, unless I lost count. About 14 %. There's a lot unplayed games in my account, though.
What's not very surprising is that the list is mostly comprised of games I played back in the day and that I still like. Theres only a few titles I discovered on GOG that I'd consider essential, namely the Fallouts and Blood.

I have a very narrow definition of "essential" ("would REALLY miss", "part of my life and my history", and "feel emotional about" do not qualify - "required for survival" does), so I'd have to say: none.

Hard to say because I haven't played through everything yet, not even closely.

If all of my archives were suddenly gone and I would be left without any options to re-obtain the games, then out of what I have on GoG, I would feel a pang to forever stay without these (in no particular order):

• Fallout;
• Fallout 2;
• Jagged Alliance;
• Little Big Adventure;
• Ultima Underworld I & II.

5 out of 47; or 10,63829787 % out of everything paid-for on my digital shelf.

The only really essential GOGs I have are

Alpha Centauri
Baldur's Gate 2
Icewind Dale 2
Freespace 2
Temple Of Elemental Evil
Planescape: Torment

Everything else in my GOG collection could be replaced at other distributors I use.

I've actually got two games in my wishlist that I consider to be unmissable GOGs, even though money troubles mean that I haven't been able to buy them yet (The Guild and King Of Dragon Pass).

Surprised to see all those isometric RPGs but not Arcanum!

out of 130 games I would say..125 of them are essential for me.

Right now I have 88 games on my GOG shelf. What are my essentials? Hard to tell.

The only one where there's absolutely now doubt would be Duke Nukem 3D. It shaped my taste and preferences for a particular style of games (3D shooters) as well as humor (non-PC-in-your-face). Another one I'm strongly dedicated to is Prince of Persia: Warrior Within. It blew my mind when it came out, more than Sands of Time, because of its dark theme, gripping atmosphere and intense story. To me it's the best PoP game of them all. More honorable mentions are MDK, Broken Sword, Abe's Oddysee, Redneck Rampage and Duke Nukem: Manhattan Project.

There are others which I'd count as essentials that are not on my GOG shelf (yet), like the first Commandos, the first Tomb Raider or the first Far Cry. Oh, and not to forget Giants: Citizen Kabuto.

That's because I haven't played it yet.

I hope that, once my backlog is a bit more manageable, I'll actually have the time and the motivation to play an RPG again. Heck, even the two Witcher games are still waiting for their first playthrough, collecting virtual dust on my shelf. :/

When you get older you are REALLY going to regret this mindset.

If I'm being honest to myself, I guess next to none. I don't think any game has really had a meaningful impact on my life and history, even though almost any game I've played is a part of them. I don't really replay games and those that I have fond memories of would stay with me even if they'd be gone from my shelf. On the other hand, if the question is how many of my GOGs I have fond and lasting memories of, then maybe it would be these:

- Planescape Torment
- Baldur's Gate 2
- Psychonauts
- Arx Fatalis
- Nox
- Neverwinter Nights (not the original campaign, but all the years spent with the community and custom content)
- Gothic (to a certain degree, mostly because of the atmospheric soundtrack)
- VTM: Redemption
- Lands of Lore: The Throne of Chaos

Maybe also:
- Beyond Good and Evil
- Enclave
- Syberia 1 & 2
- Botanicula
- Little Big Adventure
- Sanitarium
- Gobliiins

I hope to add Resonance to the list once I've actually played it.

That's about 20% of my shelf, not counting the freebies.

Out of the games I have on GOG (no particular order):

Gothic
Gothic 2
Total Annihilation
Total Annihilation: Kingdoms
Nox
Tyrian 2000
Fallout
Outcast
The Temple of Elemental Evil
The Witcher (have The Witcher 2, but haven't played it yet)
Descent 3

11/52 (21.15%)... there'll be more once I get around to playing more of them.

If I included all the retail versions I have of games available on GOG which I haven't (yet) bought on GOG, that list would be a lot longer.

Since the Infinity Engine games all still load and play well with New Systems and I have them on disc I will not list them - same goes for Witcher / Witcher 2 and others. I think I am pushing 250 GOG's.

Arcanum
Gothic Series
Lands of Lore Series
Might and Magic Series
Stone Keep
Ultima - All of them
Zork Series

Only one mention of Sacrifice. Very sad. However essential is a tricky term:

You've got the games that I really needed to play again, like Sacrifice, or Lure of the Temptress.

You've got the games I love to play, like Lords Of The Realm 2,

Then you've got the games I discovered, like Sanitarium.

Essential, none, worthwhile, all.