Maimonides held that a Ger Toshav (Resident Alien) is a legal category of "Noahide" (i.e. a Righteous Gentile) which is not applicable during the times of Jewish exile.
:"One who accepts these [basic laws] is called: Ger Toshav in every place. It is necessary to receive him in front of three Torah Scholars." (Melachim 8:10)
and againBut he also states that this category is not applicable during the times of Jewish exile:
:"And if he accepted upon himself the seven mitzvos, then he is a ger toshav, but we do not accept gerei toshav except when the Jubilee is observed, but when there is no Jubilee we accept only righteous." (Avodah Zara 10:6)
Since the time that the 10 northern Tribes of ancient Israel were exiled and lost, before the destruction of the First Temple, the Yovel years have not been able to be observed, and it is not applicable to have the Ger Toshav status. It will only be possible to again observe the Yovel years and grant the Ger Toshav status after the Messiah comes and brings the True and Complete Redemption, speedily in our days. This will be when the Messiah gathers all the Jews back to Israel, including the Ten Lost Tribes, and identifies the Tribe which each Jew belongs to. Then each Jewish Tribe will reside in the region assigned to it by G-d, as these regions are described in the Torah (see Num. 34:1-15, Deut. 33:22-23, 34:1-4).<ref>This section is partly based on the Chabad based [http://www.asknoah.org/HTML%5Cq&a_general.html AskNoah.org]</ref>
