Isn't that d'Rabbanan since Dovid Hamelech was gazar on them?
Tzvi Levi Sherman, Manlapan, New Jersey
Below, please find what we wrote regarding this issue in our Background to the Daf.
Kollel Iyun Hadaf
(a) In the times of Yehoshua, the Giv'onim (members of the Chivi nation, one of the seven nations whom the Jewish People were commanded to destroy upon entering Eretz Yisrael) came and presented themselves before Yehoshua as if they came from a far-off land. Since they claimed not to be residents of Eretz Yisrael, they requested to be converted and to make peace with the Jewish People. After Yehoshua agreed to accept them, it was discovered that they were one of the seven prohibited nations. Having already accepted them, Yehoshua did not want to break his oath and covenant with them (even though they tricked him and the oath was uttered in error) so as not to cause a Chilul HaSh-m. Yehoshua accepted them and appointed them to be woodchoppers and water drawers to supply the needs for the sacrificial service on the Mizbe'ach (Yehoshua 9:3-27). In the times of Moshe Rabeinu Giv'onim also came to be converted as they did in the times of Yehoshua, and Moshe also made them woodchoppers and water drawers (Yevamos 79a, based on Devarim 29:10). These people became known as "Nesinim," (from the root "Nasan," to give) since they were "given over" by Moshe and Yehoshua ["va'Yitenem..." - "And he appointed them..." (Yehoshua 9:27)] to perform the tasks of chopping wood and drawing water.
(b) The Nesinim are not permitted to marry someone who was born Jewish, just like Mamzerim. RASHI and TOSFOS (Kesuvos 29a and elsewhere) argue as to whether they are prohibited mid'Oraisa or mid'Rabanan. We find that the Gemara (Yevamos 79a) states that Moshe Rabeinu "decreed" regarding the Nesinim of his generation, and Yehoshua extended the "decree" to last as long as the Mishkan or Beis ha'Mikdash would stand. David ha'Melech later extended the "decree" to include all time, even if the Beis ha'Mikdash would be destroyed (because of the trait of cruelty that the Nesinim exhibited, which showed that they were not worthy of uniting with the descendants of Avraham, Yitzchak and Yakov). According to Rashi, these decrees were prohibitions against marriage, and as such the prohibition against marrying Nesinim is an Isur mid'Rabanan. According to Tosfos, these decrees were appointments of servitude. The prohibition against marrying them, though, is mid'Oraisa, since the Torah commands against marrying the seven prohibited nations even if they convert to Judaism (Yevamos 76a).