The very first diesel engine was designed to run on gasoline. That engine is not in our tractors. Similarly, old diesel engines sometimes incorporated an ether injection system for cold starts. Those engines are not in our tractors either. For example, the old WD40 had a 16: or 17:1 compression ratio -- well below the 22:1 of the OP's engine -- and was designed to start on high-octane gasoline, switching over to diesel once warmed up. So what some farmer got away with 50 years ago does not necessarily apply to today's diesel engines.
Diesel engines inject fuel under high pressure (in the case of the OP's tractor, about 2,000 psi) into the combustion chamber as the piston nears the top end of it's stroke. At those pressures, the fuel auto-ignites so engine timing is determined by the injectors. Gas engine ignition is controlled by a precisely timed spark since gasoline is more volatile with a lower flash point, requiring much lower compression pressures.
The flame in an gas engine can propagate at 90 to 120 m/s. This is considerably faster than the fuel leaves the injectors in a diesel engine, so assuming both fuels begin to ignite at the exact moment of injection, gasoline is completely combusted long before diesel would be. Combustion chamber pressures are dramatically higher because most or all of that combustion takes place while the piston is still rising. The designed engine timing is dependent upon the slower combustion rate of the diesel.
So: your tractor, your choice. If you want to put gasoline in it, then that's up to you. But don't expect Kubota to warranty it.
