Wow that end gap quote is huge for a GasGas. I have always known them to be .3mm to about .375 on the 280 and 300's. I would clean up the piston skirt, Pop a base gasket on and run it. As you can see its not a horrible job to get to and Pistons come with rings but it will be advised to do the whole cylinder piston and rings as a kit. I would run it first with a new base gasket after you clean up the piston. All you have to lose is a base gasket at this point.
--Biff