When we played Gamma World on Sunday, our GM - andi - ran Gamma World with a very light set of rules.
When I run our DF game, I run it fairly light for my game group, but it's still a heavy set of rules compared to how Gamma World played.
In my previous fantasy game, we used many more rules than now, mostly from GURPS Martial Arts and all the advanced options from Basic Set.
At the core, though, it was all the same game. All that really changed was, how many modifiers and special cases do we have matter (modifiers) or apply (special cases)?
You can set those as: Few, many, most.
Still, it was GURPS. 3d6, roll low to do things, roll high for effect.
GURPS did all of them quite well.
Our GM, andi, remarked on the rules during an email discussion:
"I was reminded again that GURPS has the versatility to allow a fast-paced science-fantasy action-adventure to be run smoothly and effortlessly, combining an old-school feel without the goofy abstractions of AD&D or the original GAMMA WORLD. Not that it's any surprise to anyone here, but GURPS is just about perfect."
One thing that one of the Mikes, who ran "Barbie," noted was that it felt like at any time, we could dial up the rules focus if we needed to, then dial it right back down to light-and-fast with no issues.
"Even with the bullets flying fast and furious, the GURPS rules set we had in place never got in the way or felt at all inadequate for our needs. But at the same time, it wouldn't have been any effort at all to turn up the detail if and when circumstances needed it."
I have to agree - when it mattered, like in the brawl vs. the Hoop, we dialed up the combat rules just a little more. When it wasn't so critical (like, acing surprised Badders in their den), we just rolled and killed and moved on. You can dial up, and then dial back down, the complexity as the needs and mood of the game session and players go. You aren't constrained by the rules, but rather the rules detail level is a useful slave to the flow of the game session.
I'd be curious to see what andi's rule set looked like. He had a copy of the free rulebook GURPS Lite, he used Action 2, and that was that as far as I know.
It really was perfect for post-apocalypse play. The rules basically faded to the background, and never got in the way of play, caused player actions that clashed with what you'd actually expect, or otherwise dominated play in any way.
In other words, if you read our Gamma World summaries, it's not really clear what the rule set is except in the rare moments where I bust out a GURPS rule explanation. If I'd said it was 1st/2nd edition Gamma World, it might have seemed odd that guys went down in single shots, but otherwise, can you tell the system without me telling you the system? Probably not especially.
There were some factors that caused this:
The GM used a very light set of rules. Much was abstracted - for example, all of our similar 7.62mm guns did about the same damage. Range bands meant that individual yardage wasn't important. Fatigue wasn't tracked except in broad strokes ("You're tired.") Things like that.
GM and Player familiarity with the rules. This was a minor factor, though - it was rare any rules questions came up, and the fact that the table had all players who knew GURPS well meant that almost nothing needing looking up (We looked up something once in two sessions - grenade and knife throwing distances.) There is a perk to running combat with someone who co-wrote most of the optional GURPS combat details. Even so, I'll cheerfully ignore all of them without hesitation, or tell the GM how they work right then and there. Whichever makes the game go more smoothly. The same players I have who'll watch like a hawk to minor rules effects will also cheerfully ignore all of it the second the GM says "We're not using that rule this game."
The inherent lightness of the GURPS mechanic. Roll low for success, high for effect, always 3d - it's a very simple game to actually play, reputation for gearheady/math-heaviness or no. It's fast, it's light, and it moves quickly if you let it. Bolt on all of the options, and yes, it slows down, but so does any game I've played when you bolt on all of the options. With few options on, 3d6 roll-under against scores largely on the sheet in front of you is fast and easy.
It was a good balance of GM skill, player buy-in, and a flexible and effective system that gives believable results.
Some of the stripped down nature of the game resulted in humor - when the player of Caveman asked if he could roll Tactics to evaluate a firing position, andi noted we didn't have Tactics because he wasn't going to require rolls against it. Caveman's player quipped, "We don't even have Soldier skill."
Queue the flurry of "Oh, we're Special operators. No wonder one of us is Short Bus!" jokes.
But honestly, as the GM said, he wasn't going to make anyone roll against it, and preferred some basic assumptions of tactical/soldierly competency and player skill (although he didn't say that in so many words.)( So it wasn't necessary. No point in focusing on something you weren't going to make important.
But as much as you could dial up the options for a game, and enforce hundreds of little rules, you can just as easily run GURPS with very little besides the basic mechanic and still have it basically be the same game. It's still compatible, it's still fast, and it's flexible to a degree I think people who haven't played it just don't realize.
I know it's not the experience everyone has, though, and I'll try to get more GURPS 101 and basic explanations of how to do things in GURPS up to show everyone just how simple it is.
I'm just glad it's a system I discovered early and which I get to play even when I'm playing and not GMing.