Is the ger toshav a convert

This piece explores if a ger toshav is a quasi jew or not

It is typically assumed that non jews are bound by the Noahide laws even without committing to live in accordance with them. So, for example, holds Rambam (melachim 8:10-11). 

What differentiates a ger toshav (a non jew that is permitted residency in israel, along with some other benefits) from a regular non jew is that he formally accepts them in front of 3 chaveirim (Rambam requires him to accept them by dint of Sinai's revelation). If he ceases practicing or shows his acceptance wasn't ever sincere he presumably has his visa revoked and is bounced from the country. He is still obligated in the noahide laws but no longer has the status of a ger toshav along with all the benefits that it confers. 

Rashba (toras habayis, bayis 5, shaar 4) has a girsa in the baraisa (tb az 64b) about what it takes to become a ger toshav that is different from our talmudic texts. Our text has איזהו גר תושב כל שקיבל עליו בפני ג׳ חברים שלא לעבוד עבודה זרה דברי ר״מ וחכ״א כל שקיבל עליו שבע מצות שקבלו עליהם בני נח אחרים אומרים אלו לא באו לכלל גר תושב אלא איזהו גר תושב זה גר אוכל נבילות שקבל עליו לקיים כל מצות האמורות בתורה חוץ מאיסור נבילות, whereas Rashba has אלו ואלו לא באו לכלל גר דגר שנתגייר כקטן שנולד דמי with the connotation being that there is a real geirus occurring as ostensibly only that can trigger such a fundamental status change. 

There's a passage in the talmud (bt bk 38a, az 2b-3a) that states that as the non jews weren't observing the 7 laws HaShem repealed the obligation. The gemara has a hard time with that concluding that their schar is only on the level of אינו מצווה ועושה. Rashi explains that they aren't obligated but still receive punishment if they transgress (limited upside, all the downside). 

According to that when a non jew accepts the Noahide strictures he is actually obligating himself in commandments he otherwise would not have been bound by. 

It then makes sense to refer to it as a conversion (into a ben noach) warranting the application of legal rebirth. 

(Apparently this geirus is effected with kabalas hamitzvos only without any need for milah and tevilah.) 

Should the newly minted ger toshav renege on his commitment it is possible that we would apply the concept of אף על פי שחטא—ישראל הוא with his non observance not unwinding his status as a full fledged ben noach although presumably he would lose his ger toshav privileges which also depend upon fulfillment of the laws.

Until this point I've been assuming that if geirus is indeed applicable to the ger toshav it is to the religion of "Noach" but not to yahadus as that requires acceptance of all the commandments to gain admittance.

However theres a comment in the peirush attributed to Rabeinu Gershom to kerisos (9a) where he explains the opinion in the gemara there that limits a ger toshav's melacha on shabbos to what a jew can do on chol hamoed as follows כישראל בחולו של מועד – כלומר כיון שיש בו קצת גירות אינו עושה כל מלאכה בשבת אא״כ אין לו מה יאכל או דבר האבד which seems to indicate (if not overreading as he may just mean as opposed to the heavier and more full throated jewish version) that geirus to Judaism isn't an all or nothing affair but rather that there are gradations within the institution of geirus with partial geirus being a legitimate option. There is apparently a spectrum of commitment to judaism with renouncing az (so holds the peirush there) on one end and full acceptance of the entire torah on the other. 

It (the ger toshav's status as a quasi jew acquired with accepting monotheism) jives nicely with az being referred to as (bamidbar 15:22) וְכִ֣י תִשְׁגּ֔וּ וְלֹ֣א תַעֲשׂ֔וּ אֵ֥ת כׇּל⁠־הַמִּצְוֺ֖ת הָאֵ֑לֶּה with Rashi commenting מצוה אחת שהיא ככל המצות. Ditto for the gemara in makkos (24a) בָּא חֲבַקּוּק וְהֶעֱמִידָן עַל אַחַת, שֶׁנֶּאֱמַר: {חבקוק ב׳:ד׳} ״וְצַדִּיק בֶּאֱמוּנָתוֹ יִחְיֶה.״ which too suggests that the mitzvah that best encapsulates judaism is belief in HaShem. If judaism is primarily about the acceptance of one deity, then it makes sense that it starts one down a path of conversion and gets the ger toshav a foot in the door. 

A gemara (b meg 13a) is fairly explicit about it: כׇּל הַכּוֹפֵר בַּעֲבוֹדָה זָרָה נִקְרָא יְהוּדִי.

(See this piece https://musingsonthetorah.blogspot.com/2023/02/when-did-jews-become-jewish.html where Ramban seems to adopt a vision of Avraham as the first jew albeit with limited commandments, apparently the meaning of being jewish isn't necessarily tied to being obligated in the entirety of the Torah)