Wow, that Starion is wicked. I have a buddy who would dig that and do it if someone can get the owners board name, need to pick his brain on that one.
Well, I feel bad for you because you sent your GN to a GN specialist and it left in worse condition, not right IMO.
It looks like you bought it and dropped it off with a stock to rock setup and this is what comes to mind, in concern to the blown head gasket, after seeing the receipt.
They messed with the Wastegate Actuator so they had to tune the boost by adjusting the rod ( it said HD so it's the 18psi one ). This would tell me that they have to go WOT with it to see what length they need the rod to be in conjunction with what the chips boost setting is.
So, if you still have a stock pump and it spikes to say 20psi on pump gas that could be it right there, done. Then you get in it, hit it to WOT and see it smoking like Cheech and Chong and then it's a gauges fault??
The only gauge that tends to be more sensitive then the ALDL reading is the Caspers knock gauge. If you don't have a Scanmaster get one asap so you know what is going on and can slowly take care of some basic stuff on your own.
Huh, the place is called White Racing and you have a blown head gasket after them tuning your car.
Stick around here and search as much as you can and you will feel better about picking up a tool or two and working on the GN.