As much as I'd like to say it's easy to learn, it isn't.
The basic of the system is very easy- roll 3d6, and check it against a target number. If it's under the target number you succeed.
It gets hairier when you move to character creation, and more complicated still when you move on to combat. There are basically rules covering nearly every situation, including situations you might not be particularly concerned about or think you need rules for.
If you want to learn it, then you really have to want to. A casual read through might let you play in a game someone else is GMing, but it won't let you GM and it won't give you access to the more rewarding optional rules.
The good news is once you learn GURPS, and if you really like the system, you won't have to learn anything else. The Generic Universal part of the name is not hyperbole, you can honestly use it for any kind of setting, or even better, go crazy with time travel and dimension hopping and mix all your favorite settings together on a combo platter.
The biggest flaw is that the system by default tends towards realism, and has a lot of optional rules to make it even MORE realistic, while contrarily having less development in rules to make the game more cinematic. So if somebody shoots you with a gun or hacks open your chest with a sword, and you aren't wearing the appropriate armor, you'll likely end up passing out after a few seconds of activity if you aren't killed outright. (Then again if you've statted yourself out as The Hulk instead of Joe normal, you'll probably be completely unharmed). I've discovered many ways to tweak the system in favor of high action games, but it required a lot of experience and experimentation.