Both HPTuners and DHP PowrTuner are able to tune 3.1 GP's, since they were part of the initial wave of cars looked at by them as they got started.
As far as "is it worth it"? Not really, unless it's your hobby vehicle you want to squeeze a little more from. Since the 90's, a big wave of modders threw everything they had at the 60deg V6 engines, and the gains were never too impressive. It may be the fault of the engine design being unable to support increased power reliably, despite estimates that the geometry "should" out-rev and outperform our 90deg V6's being pushed over 2-3x stock power levels.
For your personal needs, do what you need to for keeping your car as nice as you want. There's still a dedicated community for that family of engines, as there is for 3800's, but both are shadows of their early 2000's glory.
