OP - I left you my cell number in a private message. We can help.
I can guess who provided that quote because we've seen similar quotes for related work.
If same dealership I'm thinking of, the GT owner took his GT in for the airbag recall. Despite having an appt, the dealership incurred a delay and kept car overnight. The next day they said they were done. He picked it up and drove away. Within a block, no HVAC. No fan, no AC, dead controls. He turned around. Dealership claimed it was unrelated to their airbag work - but they would take a look anyway (since they are so nice). Almost 2 hours later they diagnosed problem to be faulty HVAC control module and quoted him "about $6000" to get and install part.
Thankfully, owner was smart enough not to trust them. Over time, he found us and told me the story. I told him that the HVAC was literally immediately adjacent to the passenger side airbag and I sincerely doubted it was bad - but odds are they did something wrong. Turns out we were right (kudos to Ryan for isolating problem). The dealership had inadvertently (hopefully) bent a pin in an electrical connector coming from the console HVAC controls to the module. Ah! Found it! Not so fast. We fixed the pin and connector and still the HVAC did not work. As it turns our there was another connector that simply was not connected. Once connected, everything was back to perfect.
In the best case, it was just real sloppy work combined with very poor diagnostic skills. In the worst case, well, let's not go there.
Gauge problems are EZ to diagnose and resolve.