As to why there are few full sources, the Jewish view is reflected as in this excerpt from the Shema Yisrael Torah Network website:
There are a few reasons why there are so few contemporary sources about the ger tzedek story. It can be assumed that the noble Pototzki family, which was a religious Polish-Catholic family, was not happy that one of their sons defected to Judaism. The Pototzki family was said to have generally dealt kindly with the Jews living on its lands. Mentioning the conversion would have been interpreted as an open provocation of the area's ruler, which would have not resulted in any good. In addition, undoubtedly the conversion of one of the upper-class gentiles aroused great interest among the populace, and his refusal to return to their faith caused them great embarrassment ... Nevertheless, we believe the words of our rabbis, which clearly indicate that there was a connection between the Gra (i.e. the Vilna Gaon) and the Ger Tzedek