Remove it and take the tank unit out and if it isn't too bad you can get rags in and work them with screwdrivers and get it clean. If it is too bad for that, take it to a radiator shop as they can boil it out.
The easiest way is to syphon all the fuel from the tank using a old second hand fuel pump.
You will need minimum, 1 old 12v fuel pump, get from a scrap yard etc. 15ft fuel hose that will fit both...
Every motorcycle I've ever seen for the past 30 plus years has had a removable gas tank, unless it's a custom built chopper with the tank moulded/welded to the frame.
But if you want to do it the...