New to GURPS and concerned about combat

[Kuroshima]

There is one truism that remember, GURPS is like a cow, you don't try to eat the whole thing at once. GURPS is also front loaded, in that you do most of the work during character creation.

Given that you come from a D&D background, I recommend that you pick at least Dungeon Fantasy 1 and 2. You will both see how to adapt GURPS to a familiar genre (including what options to use and what options to discard), you will also get a set of templates to help you with character creation.

On using psi instead of magic, just as with everything in GURPS, you have multiple options. The basic set version of psionics is more or less the starting point of all of them. GURPS Psionic Powers expands it in one direction, GURPS Dungeon Fantasy 14: Psi goes in another. Both are ready to use with minimal assembly required. GURPS Powers gives you more tools to tinker with them. Still the basic magic system might suit you too.

Oh and something that you might not realize right away, on top of the books with GURPS in their title, the Pyramid magazine is another source of materials for your game, as official as any other.