(1) The title "Gazlan" certainly does not remain with him forever, as the Rambam writes in Hilchos Teshuvah 3:14: "There is nothing that can stand against Teshuvah. Even if someone denied G-d for his whole life, and just before he died he did Teshuvah, he will receive a portion in the World to Come."

(2) However, the Gemara does not say explicitly what exactly is defined as Teshuvah for a Gazlan. We can learn some of the principles of these Halachos, though, from the Gemara in Sanhedrin 25b. For instance, we find there that people who received interest in return for their loans may not serve as witnesses until they tear up the deeds of these forbidden loans and their is strong evidence that they have done complete repentance and will not return to their old ways.

(3) The Rambam in Hilchos Eidus 10:4 writes that even though a thief returned the stolen money, he still remains disqualified for testimony. The Tur (CM 34:15) writes that he does not know why the thief should not become a valid witness once he has returned the money. The Tur suggests that the Rambam means that although the thief returned the stolen money, if he did so only because he was forced by Beis Din to do so, he remains an invalid witness until he returns the money of his own free will.

(4) The Beis Yosef there is slightly more stringent and writes that even if the thief returns the money on his own initiative, he regains his status as a valid witness only if we are convinced from his new behavior that he has done Teshuvah and will not steal again. This is the ruling of the Shulchan Aruch (CM 34:7), who writes that we must know that the thief has done Teshuvah.

(5) The Rema (CM 34:29) adds that the above applies only to a thief who often and consistently stole. If someone only stole occasionally, as soon as he returns the stolen items he regains his status as a valid witness, assuming he returned the goods of his own volition without being forced to do so by Beis Din.

(6) In summary, as soon as it is clear that the Gazlan has done a genuine Teshuvah, he loses permanently his title of "Gazlan."

Dovid Bloom