First, how do you KNOW that John's epistle is dealing with the false teaching of the Gnostics. It does not say THAT. It does deal with false teachers, but the idea that they were Gnostics is something that has to be derived from other than the scriptural record. And, in fact, giving the Gnostics such an early dating can be problematic (read "The Gospel and the Greeks" by Ronald Nash for a bigger picture of why that is an issue).
And, second, if John is not writing about the sin of a believer, then what on earth is he actually writing about. I am of a mind that you are considerably clouded as to your interpretation of John's epistle, and if so with that simple book, likely more so with the rest of Scripture that proves more difficult. In that ALL unbelievers are sinners, and all believers are yet sinners, though their sin (singular) has been forgiven, John is not just writing to unbelievers. Hence his address to the church, and not to the lost world around the church.
Essentially, according to what John wrote to the believers, you sir are a liar. Note that John says that. I am just pasting his words. I mean no personal offense.