"Possibly those were two of the worst failures on this model and John Deere had them do that?"
John Deere doesn't make the dealer do anything with respect to fixing a used Gator unless there was a safety recall. After the short warranty period is up, JD doesn't care if anything breaks as they have a virtual lock on the replacement parts and make considerable revenue from it.
My guess and that's all it can be, is there were considerable issues with this machine making it impossible for them to sell it without fixing it.
The head gasket didn't just go bad without some other issue occurring, such as over heating. The shifting arms likely broke because the clutch didn't release as it should. etc.... Are they willing to warranty the repair work?
Only you will be able to determine if the price to purchase risk is at the threshold to move forward or not. Any purchase is a risk.
The idea of using it for a longer period of time to verify its state of operation sounds like a great idea. A good thorough inspection to check for leaks, bad bearings/bushings and other operational performance parameters is encouraged. JD Parts are expensive as they go to great lengths to ensure a lack of competition.
2012 JD855D OPS Black Poly Roof, Deluxe Cargo Box, Front and Rear Protection, Power Lift, Front Hood Rack, Rear Screen (homemade), 800 Watt Inverter, 2500lb Winch installed in cargo bed, Front and Rear LED auxiliary lighting (4" square 27 watt), Front and Rear CV Guards, Koplin Side Mirrors