If you do drain the tank, be sure to have plenty of empty containers on hand. There always seems to be much more gas in the tank than what you expect and you don't want to have gas running all over you and your garage floor while you frantically try to thread the plug back in. I learned that lesson the hard way.
You will end up with headaches if you don't get it cleaned out manually. Sorry - but you can comb through the threads and read about gas tank/line troubles from here to kingdom come. Like Jshcobra states - be careful and you can slowly use it up in your dd a few pints at a time to get rid of it. It will dilute it enough. You will want to back blow the lines if you can before you drain it - also check for rust!!! If you have chunks in there it's in the lines too and you may as well flush or replace. then you are going to want to rebuild the carb if it got varnished up and giver a good cleaning/soaking in a bath of cleaner. Good luck. nip it in the bud now and you will be golden if you follow it up with a can of seafoam every now and then to maintian it.