Gamerscore resets are only given if you've been found to have cheated/modded your gamerscore. Someone would have to file a complaint on you via the console and the enforcement team would reset you after an investigation. You would never be able to reearn
any of the achievements.
If you have legitimately earned your gamerscore, you would have to start a new account in order to have 0G. There is no way to remove gamerscore.
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