I would not think that the GMPP tune is what a dealer would call a tune. It is a GM option. If you told them that your car is tuned they may not understand. The bottom line is that if they go to TIS2WEB to re-flash your ECM, your tune will be there. It isn't like a Trifecta tune that they can't work with.
The wreck could have knocked something loose, either a hose or electrical connector. It could have also cracked something like the intercooler.
I would try that or another GM dealer again. Or an independent shop that is certified for GM Powertrain.
Sky NA 2007 Midnight Blue
Sky RL 2008 Carbon Flash - Naked