If you can get your boat up/on a lift/out of the water then fairly easy to get the siphon to pull the fuel out.....some clear plastic tubing from Home Depot would work...
I bought a syphon pump from Wallmart. About 7 .00$ . Worked like a charm. I doubt you can just gravity syphon due to the anti syphon valve in the tank.
Took about twenty minutes to remove 35 gallons. I put the old gas in my truck in 5 gallons per fill-up. This was from a boat that had not been used since 2011. No problems. Dave-R