Accepting rebirth involves two steps: repentance and, as a natural consequence, baptism.
History: There is not enough information about the nature of baptism in the Book of Mormon to know whether Alma’s listeners had already been baptized. I hypothesize that baptism was not a “once in a lifetime” proposition during Book of Mormon times. The Jewish practice to which it is most similar was a full body washing that could be done at any point in one’s life when a recommitment was desired. In pioneer times, rebaptism as a renewal of commitment was commonly practiced in Utah, although that practice is now discontinued. (See commentary accompanying Alma 4:4.)
Because the Gideonites are righteous, we might suppose that they had already been baptized. However, the people in Gideon had probably arrived with Limhi and Ammon—like Alma1, refugees from Lehi-Nephi, but leaving at a different time. Alma1 did not begin baptizing until he had left Lehi-Nephi. Limhi had requested baptism of Ammon, but Ammon declined (Mosiah 21:33). It is therefore possible that the Gideonites might not yet have been baptized, even though they were believers in the Nephite religion and the Atoning Messiah.