Not going to happen. Ever. A GTO (Holden/Vauxhall Monaro for those of you not in the US) is a completely different car. The show cars at LMA weigh only 1400 or so pounds and are powered by a 150 HP mid mounted motorcycle engine, whereas a (albeit stock) GTO weighs in at over 3700 pounds and has a 400hp V8 mounted in the front. It would be nearly impossible to replicate the stunts in LMA with a GTO (unless they put fiberglass GTO bodies on the current cars). The LMA stunt cars are light, nimble and I'd be willing to bet have 50/50 weight distribution whereas a (stock) GTO has 55/45 weight distro (not conducive to anything but powerslides). There are about 100 more reasons that it will never happen but I'll leave with the most compelling of all:
Why would they replace the Opels (which are still sold in Europe) with a car that isn't sold in America after the 2006 model year?