I am an Italian noahide.
My question expresses a purely intellectual curiosity, concerning the Jews and not the Gentiles.
As we know Rambam states in Hilchot Melachim 4: 4:
“A commoner is forbidden to have a pilegesh; but he may have an “amah ivriyah”, after she was designated (by her master)”
This is how HaGra expresses himself in his commentary on Shulchan Aruch, Even HaEzer 26: 1: 6-7
“So writes Rambam in chapter 4 of Laws of Kings [halakha 4], that she (the pilegesh) is not permitted except to a king, but not to a commoner. Ramban criticized this, [in the Responsa of Rashba attributed to Ramban, #284] based on the fact that Caleb, Gideon and other great men of Israel all took pilagshim; and if the pilegesh of Giv'ah [Judges 20] was forbidden, he would not have said publically before all Israel "My pilegesh" [ibid. verse 4], and would not have gone publically to her father. But Rambam implicitly replied to this by writing, that a person who is not a king is permitted only to take an “amah ivriyah”, and would explain that such was the case re all the above [commoners who took pilagshim]”
According to HaGra, therefore, if I have not misunderstood, for Rambam the pilagshim mentioned in the Tanakh as partners of Jewish commoners would all be examples of amah ivriyah designated by her master (see Smemot 21: 7-11). However, I did not understand if this interpretation is the result of HaGra's brilliant solution aimed at reconciling Rambam's position with the references in the Tanakh to the pilagshim taken by Jewish commoners, or if we actually have textual evidences, in Jewish sources, about the fact that an "amah ivriyah" designated by her master was called a pilegesh, and especially if the pilagshim taken by Jewish commoners, which are spoken of in the Tanakh, were all cases of amah ivriyah designated by his master (it does not seem to me that the Tanakh and the Talmud express themselves in this sense, but obviously I am ignorant of the Jewish tradition as a whole)