The answer to your question goes across the alleged discrepancy between Lev 17:10 – that forbids the ‘sojourner, stranger’ (GR) to eat blood and the passage you speak on (Deu 14:21), which says that to the ‘sojourner [or, ‘stranger’] inside your doors’ (LGR AŠR BŠORIK) could be sold unbled meat. (Your mention of גר תושב (GR TUŠB) is drawn – probably – from the passage of Exo 12:45 that does order that a TŠB cannot eat the Passover sacrifice. Interestingly, the previous verse (44) seems to indicate that one factor that distinguishes was the performing- or not performing the circumcision.)
Jewish scholars, and other Bible commentators have recognized that the distinction must have been the religious standing of the alien involved. Sometimes the term GR meant a person among the Israelites who was not a full proselyte. It appears that this sort of person is meant at Deuteronomy 14:21, a man who was not trying to keep all of God’s laws and who might have his own uses for a carcass considered unclean by Israelites and proselytes.
According John Gill, the GR that was permitted buy (and, to eat) unbled meat was (bold is mine) “not to the proselyte of righteousness, for he might not eat of it any more than an Israelite, and if he did, he was obliged to wash his clothes, and bathe himself in water, and was unclean until the evening, as in Lev 17:15 but to a proselyte of the gate, who took upon him, as Jarchi observes, not to serve idols, one that has renounced idolatry, but has not embraced the Jewish religion; such an one might eat of things that died of themselves, or were not killed in a proper manner. The Targums of Onkelos and Jonathan call him an uncircumcised stranger or proselyte, who had not submitted to circumcision, as the proselyte of righteousness did.” [on Deu 14:21]
The Pentateuch and Haftorahs, edited by Dr. J. Hertz, observes (bold is mine): “According to Lev. XVII, 15, touching or eating the flesh of a nevelah is defiling both to the Israelite and the ‘stranger [or alien resident].’ In Lev[iticus] the ‘stranger’ meant the non-Israelite who had become a proselyte in the full sense of the word, a ger tzedek [‘stranger of justice’]. Here [in Deuteronomy 14:21] the ‘stranger that is within thy gates’ refers to the time when Israel would be settled in their Land and would have in their midst not only proselytes, but also men who while they had abandoned idolatry did not completely take upon themselves the life and religious practices of the Israelite. […]” This work explains again that the ‘stranger’ (alien) of Leviticus 17:15 was “a full proselyte, . . . otherwise, he was not debarred from eating it.”
Matthew Henry sustains (bold is mine): “Now as to all these precepts concerning their food, [1.] It is plain in the law itself that they belonged only to the Jews, and were not moral, nor of perpetual use, because not of universal obligation; for what they might not eat themselves they might give to a stranger, a proselyte of the gate, that had renounced idolatry, and therefore was permitted to live among them, though not circumcised; or they might sell it to an alien, a mere Gentile, that came into their country for trade, but might not settle it, Deu 14:21. They might feed upon that which an Israelite might not touch, which is a plain instance of their peculiarity, and their being a holy people.”
I hope this will help you.