How do you become a GM on WoW

Go home
you have to be able to multi task cause you are actually talking[typing] to about 5 ppl at once tyring to solve their problems
you have to know what you can and cant do about their problems alot of technicalities will arise no doubt you caused some of them e.g 60%disputes, ppl stuck, pp cant hand in/accept quests, bosses not dying or droping loot = you have to know all of it.
you have to be over 18 oops sorry to all the teens that thought they could make a gm char and make him op and gank all the ppl who gave you crap + camped you while you were leveling by droping infernals all at once in the prince fight in kara. Or one shot everyone in bgs on your day off = ppl will report you.
you cant be a gm and play wow at the same time once again srry to all the hopful teens \cry nor can you give all your friends large amounts of gold, items,raid help well you can if you don't want to keep your job very long. Watching ppl play wow when you have to answer little teens oh wait that's you! stupid Q's all day can really be frustrating.
oh you still have to pay for your account and exp, you might get an employee discount of not much.
oh wait there's more but i thought this will be enough to put anyone off who would be applying for the wrong reason. Ask yourself is it worth it ?
-----------------------------------------------------------------
You can be a GM on a WoW Private server multiple ways.
But to be a Blizzard GM you must apply at the website for a job.