I see no point of getting it on these trucks since they are tried and true for almost 20 years. If you think about it, not much has really changed on these. Sure, little bits here and there such as the 350 bring replaced but the 325 (pretty much the same engine anyways), coil spring vs torsion beam, etc.
I did, however, purchase GMPP for my last car; 2006 Chevy Cobalt SS Supercharged. It was the 2nd model year of a first generation platform and engine family. The transmission had been around for a while (Saab 9-3), but not really tested for the performance this car had, not the final drive gearing of 4.05. This purchase paid off big time. New trans, new pistons, arms, main bearings, crank, timing tensioners, oil pump, wheel bearings, and a few other things. GMPP saved me thousands.
I just think the question that should be asked is how new is the technology in the vehicle. If it's something brand new, get it. If not, don't bother. Judy my opinion.