There is a Torah obligation to immerse utensils that one acquires from a gentile. The Torah requires immersing utensils in "Meh Nida" ("Nida waters"), referring to the waters in which a Nida immerses, namely, a Mikveh consisting of at least forty "Se’a" of water. Even though the utensil is brand new, has never been used, and thus obviously has no particles of forbidden food in its walls, it must be immersed. This obligation has nothing to do with ‘Hag’ala," purging utensils of forbidden food particles that it had absorbed. Rather, it is required by virtue of the fact that the utensil is "upgraded," leaving the possession of a non-Jew into the possession of a Jew. Just as a convert immerses in a Mikveh as part of his ascent from the status of a non-Jew to that of a Jew, similarly, a utensil that leaves the possession of a non-Jew and enters the possession of a Jew requires immersion.
Hacham Ovadia Yosef, in his work Yabia Omer, addresses the question of whether this obligation requires a convert to immerse all his utensils after converting to Judaism. Before his conversion, of course, he was under no obligation to immerse his utensils, since this Misva does not apply to gentiles. Now that he converts, his utensils seemingly are transferred from the possession of a non-Jew to that of a Jew, as a result of the owner’s conversion. It would thus appear, at first glance, that when the gentile returns home after his conversion he must collect all his utensils and bring them to the Mikveh for immersion.
Hacham Ovadia, however, rules otherwise. He claims that when a non-Jew converts to Judaism, his property becomes Hefker (ownerless), and he must then take possession of all his belongings anew after conversion. As such, his utensils do not transfer from the possession of a non-Jew to the possession of a Jew. Rather, they become ownerless, and then the convert – who is of course now Jewish – takes possession of them. Halacha requires immersing only utensils acquired directly from non-Jews, and not utensils acquired from a state of Hefker. Therefore, since the convert acquires these utensils from their state of Hefker, they do not require immersion.
Summary: There is a Torah obligation to immerse in a Mikveh utensils acquired from a gentile. A gentile who converts to Judaism does not have to immerse all his utensils after his conversion.
