The idea is that it provides an additional Tumble check as a free action. A normal Tumble check can be performed as a Free Action ONLY when part of a move action; if you've expended both of your actions per turn, you are granted one additional use of the Tumble feat. So, for example, you can attack, move 20 feet and Tumble past opponents normally; with Gun Fu, you can do that, and then may Tumble past opponents once more time (up to 20 feet) thanks to the feat.
It basically increases your movement range as long as you are only wielding pistols and are using the Tumble feat in the process. Cinematically, it represents some of the John Woo-esque gunfights where you have the hero rolling across the backs of his enemies, diving through their legs, rolling over boxes and sliding down rails, etc.