<Scrambles to post advice>
DON'T TRY AND READ THE WHOLE THING!
Seriously. As
nanoboy said, almost all of the rules are optional, so if you don't think you're going to have psionics in your game, for god's sake don't read chapter 6, for example. Same goes for Magic. There's too much there to take in, and you'll just overwhelm yourself. Also, just skim over the advantages, disadvantages and skills chapters. You'll be tempted to read in detail when you see interesting Ad/Disads/skills, but I recommend against it. Maybe later when you've already digested more about the system, you'll have the mental bandwidth to start getting stuck in to that.
Instead, the trait lists on page 297 are your friend. There you can see the names of all the Ads/Disads/skills/techniques/etc, along with their cost, a few other key bits of info, and the page number where that item is detailed in full. Use this to look for the sorts of things you're after, then look up the detail for ones that look like what you want.
The chapter on templates is probably only worth reading if you're going to be using character templates that someone else is supplying to you for your game. (Which is possible. One of the supplements probably has character archetypes templated up that would suit you)
Read the first few pages of the Equipment chapter, where it explains the stats, then skim the tables. Chapter 9 through to the end is good stuff, so read that.
So of the 338 pages of the first book that's pretty much pared it down to 76 pages. You could start GMing a game at this point, but you'll probably want read chapter 10 and 18 in book 2. Then you really could start playing.
Start reading the other chapters bit-by-bit, and introducing the mechanics that you like as you go. You can even tell your players that you didn't use that rule before because you were introducing them to the rules gradually. Which would be true, except that you're also doing the same for yourself.
stork also gives good advice. (Except where he said to read all of the core books
). GURPS Lite basically covers the same bases that I recommend you restrict yourself to above, but in only 32 pages.
I'd also recommend the supplement GURPS Template Toolkit 1: Characters.
www.warehouse23.com/products/gurps-template-toolkit-1-charactersIt's mostly geared towards creating character templates, as the title implies, but in doing so, it covers all the bases of how to make balanced and effective characters that fill the roles you intend them to.
As you've been using Pathfinder, you also might want to check out the Dungeon Fantasy series of supplements. They cover everything you need to know about which rules to use, and how to build characters, and generally how to use GURPS to produce that style of game.
Have fun, and don't get overwhelmed!