I always treated Psionics in GURPS (and D&D, among other RPG) as just another type of magic --- the moving of energy to create an effect. what separates Psionics from magic in the game is that psionics is permitted for certain character types where magic itself is disallowed. (Paladins in D&D for example)

In a D&D game I had a master thief character who gained Psi abilities later in the game, which he used to augment things he did through skills. It was funny as hell when the critters who were chasing him sank in the quicksand he ran across (using a psi ability to walk on the surface)

Psionics when permitted in a game is basically limited by the GM because there are so many ways it can be applied. Similar to when you mix technology and magic.

hah! funny you should say that, I'm starting to get a grasp of it, but use it rarely and the power group thing still has me a bit confused. A bud has explained it twice to me now, but the 3rd ed rules seemed to make more sense to me.