RE: GL
This kind of BS is a big part of why I've stopped paying attention to GL auctions.
Based on their (absurd, one-sided, and IMHO illegal) terms of sale, I would not be at all surprised if they banned you from bidding again. I'm not trying to condone this, but it's what I expect they'll do.
That being said, 60,000 miles on a 30+ year old truck is no big deal, even if that is its original speedometer. Since the speedometer (which contains the odometer) and the tachometer (which contains the hour meter) are both generic and easily-replaced parts, there's no guarantee that the odometer and/or hour meter readings really indicate the number of miles on the chassis or hours on the engine; they could easily by 10x higher or lower than the real numbers. And the truck probably doesn't have its original engine in it, anyway. I would never blindly trust the odometer and/or hour meter readings on any military truck, and would always go by the apparent condition of the truck after my own personal inspection.
So, I can understand why you would be upset at that discrepancy, but I doubt you'll get satisfaction from GL on it, and you'll probably lose the right to ever bid in their auctions again (maybe you already have?). Even if they had transcribed the odometer reading correctly, it still could have been a P.O.S. truck that had its speedometer replaced 6,500 miles ago. Or a non-running heap that had a 6,500-mile-old speedo stuck in it to fill the hole. Or it could really be a 6,500 mile truck and some jackass swapped the speedo with another truck after the description was written. The odometer and hour meter readings on a military truck with a long and unknown history are meaningless, because it takes a few minutes with a screwdriver and a pair of pliers to swap either meter. Even the truck's dataplate is considered a replaceable part by the military.
Unless you just want to try to get your money back and never play the GL game again, I think your best bet is to see if you can still pick up the truck, and chalk your high bid amount up as a lesson learned.