From a strictly logical perspective, your arguments are circular. Pauline sources validated a gospel, therefore it is true. The gospel is true because it reaffirms what the people who recognized it believed.

The proponent of a Gnostic vision of Christianity will counter that, had Valentinus not lost the PR war inside Christianity (by having a message that was less popular but which could theoretically have been more correct and closer to the real teachings of Jesus), we would have plenty of Valentinian sources that would renounce, say, the Gospels of Matthew and Luke, as well as the writings of Eusebius, Irenaeus, and Tertullian, as "heretical". The "Orthodox Gnostic Church" would base its claim to orthodoxy on the fact that Valentinus was taught by Paul's disciple, Theudas.