Do you need system mastery to run GURPS?
Over in his excellent wrap-up post on his GURPS game, David Larkins argues, that yes, you do.
I disagree, to a degree.
I'd argue that GURPS, like any game system*, rewards system mastery.
But it doesn't require it for all players.
It only requires it for one person - the GM.
So long as the GM is willing to make up the characters according to the player's descriptions, and the players are willing to describe their actions in real-world terms and trust the GM to adjudicate them, it will work. This doesn't even slow things down, if the GM is conversant in the system.
Case in point, I'm absolutely certain some of my 1st edition GURPS players had no idea how anything worked except "roll below your target number to attack, defend, or resist, and roll high for damage." I'd have to help them make PCs, spend points, figure encumbrance, and tell them when to roll. They just didn't care. They'd tell me what they wanted to run, and I'd help make it or just make it for them. I'd figure out what the penalties were and told them and they'd roll. Everyone seemed pretty happy, but I'm sure most of them never read the rulebooks (one did, the others, I'd bet no).
My new group is different - all but two are pretty rules fluent. Of those two, one is moderately rules fluent and the other doesn't care to learn more than the very basics. He doesn't need them - he can lean on the others for rules help, or just say "I'm doing [whatever]" and I'll rule on how it works.
Now, I'm not saying rules mastery, rules fluency, etc., doesn't help. It helps in the same way as it helps if everyone in your group knows the Labyrinth Lord rules or is can quote page refs for spell effects out of the AD&D Players Handbook from memory. It's just handy, and it speeds things along. It relieves some of the pressure of the GM of remembering it all (although it introduces rules lawyering and fights, potentially). But you could run a whole game with GURPS Lite, or even with a subset of GURPS Lite, without any real loss, if all you want is a rules system to cover the generalities of play and are willing to wing the specifics.
And as long as you keep to "roll low to do something, roll high for effect" as your guideline, you can ignore a good 90% of the rules. I know, because I have. I've skipped over a lot of rules when they just got in the way. Only a few are truly required. Skipping defense rolls will fundamentally change combat. But ignoring the rules on a combat Step vs. a combat Move, well, not so much, not if everyone is doing the same thing - it's not a game-breaker. You can toss out reaction rolls, death checks, fatigue rule, surprise and initiative, whatever. Roll low for success, roll high for effect. It'll work out. Some things work better with a few more rules, but that's fine.
You'll still want the person making the keep-or-toss decisions on rules, and running the whole show, to be game system fluent. A game master, if you will. It's part of the job. D&D was sure better when I played with someone who actually knew the rules. Ditto for everyone else I played. But it's only the one person that needs this mastery.
It just takes one person who knows what he or she is doing, and the willingness of the players to go along with it.
* Yes, even early D&D does. Know all the spells and memorized the "to hit" tables and have a precise knowledge of the odds of all of your saves? Try and convince me that doesn't help - heck, even the DMG advises you to take away magic items from the PCs if they read the DMG. Why, because it doesn't matter? Heh.