Need to drain the gas tank as I am planning on replacing all the fuel and vacuum lines.  Of course I have a full tank at the moment.

Can someone tell me what is the easiest way to drain the tank?

Thanks......

I have about 5 feet of 1/2 Diameter Tygon Tubing I use to siphon the gas from the tank into a 5 gallon container, I have already done this process 3 times this week working on the M50 (aim for the lowest point in the tank) 

   

I obviously haven't done it on a GS500, but I've drained tanks thru the carbs as well. Get a gas tank like Stevo shows and hook up a line from the float bowl drains. Open up the drain screw and turn your petcock to prime.

Could you also take the tub off the petcock and run a larger tube onto that tube?

yep, you could also pull the tank and just put the tubing directly onto the bottom petcock  (sorry if that's what's steve's picture shows, it's not loading for me  )