AnswerNot nearly as bad as the other way around. It happened on a 2000 Silverado V6. Here's how to fix it in case you ever need to do it yourself.
1. Remove the gas cap to relieve the pressure in the fuel tank. This is a must!!
2. If you have room, crawl under the vehicle and locate the fuel filter. On this vehicle, it was on the driver's side inside the frame rail of the truck.
3. Disconnect the fuel lines from the filter itself and pull them out of the filter enough for the fuel to flow out.
4. Get under the hood and remove the cover to the fuse box and find the fuel pump relay. It should be labeled in the schematics on the lid. Remove the relay and note where position 87 and 30 are located in the fuse block.
5. Make up a jumper wire that you can plug into those 2 positions and the fuel pump will start pumping the fuel from the tank. You can turn the key to the ON position only, to see the gas gauge slowly go down as the fuel drains. When you sense that you hear air, that means that very little is left in the tank. Pull the wire in the fuse panel to stop the pump.
6. Add about a gallon or so of regular gasoline to the the tank and repeat step 5 until you hear air again. This is to flush out the tank, pump, and lines of the remaining diesel.
7. Crawl back under the vehicle and reconnect the fuel lines to the filter. Make sure they are snug.
8. Add about a half gallon of gasoline to the tank.
9. Plug the relay back into the fuse block noting which position that it plugs in according to the schematic on the lid.
10. Turn the key to the on position and the pump should kick on for a few seconds. Check to be sure that there is not any fuel leaking.
11. Add about 3 or 4 gallons of gas to get the vehicle started. It will take several tries in order for the gasoline to push the excess diesel through the system.
12. When the vehicle finally does start, it will run very rough at first. This means that it is burning off the remainder of the bad fuel and the gasoline is burning it off. It should smooth out in a few minutes.
13. After the engine starts idling normally, drive the vehicle to circulate the fuel through the engine. You may see a little smoke, which is normal as you burn off the diesel.
14. Go to service station and put in a tank full of regular gasoline and drive on....