As in you can never get it back.
In addition to losing your Gamertag, you’ll also lose your Gamerscore, saved games and any points you have on the account.
While I am on the subject of things you should not do, I strongly urge you not to tamper with your Gamerscore and Achievements. If you decide to employ some nefarious techniques to artificially increase your Gamerscore or obtain achievements by manipulating the Xbox software without playing the game, bad things will happen. What type of bad things? For one thing we may remove the entire Gamerscore for an account that contains manipulated gamerscore. Let me be more clear: If you use tampered gamesaves to boost your Achievements and Gamerscore, you may sign in to Xbox LIVE one day and discover that we have removed your Gamerscore. Once we do that, you will never be able to re-earn those achievements on that account. Even worse, you might have your account and/or console banned from Xbox Live.
As you can imagine, we know how to find out if you do this and we do take action. And we’ll continue to do so.
The best way to increase your Gamerscore or gain that Achievement is to earn it like a real gamer; play the game. Then you can rightly be proud of your Gamerscore as you earned it yourself and not have to worry about anything. I imagine that the majority of you earn your Gamerscore honorably, but I know there are some people out there are may not be as honest. In short: Don’t do it. It’s not worth it.