Why not? Good question!
Your unencrypted password is never stored on our server (or at Google, for that matter), but it's still a good idea to use different passwords for different sites, or at least for sites where it would upset you to have your password stolen. This is because the most common way someone can gain access to your account is simply by guessing your password. Using different passwords ensures that, even if someone figures out your mySQLgame password, your Gmail account will still be safe.