John Gill Commentary Deuteronomy 14:21

"Ye shall not eat of anything that dieth of itself: thou mayest give it unto the sojourner that is within thy gates, that he may eat it; or thou mayest sell it unto a foreigner: for thou art a holy people unto Jehovah thy God. Thou shalt not boil a kid in its mother`s milk." — Deuteronomy 14:21 (ASV)

You shall not eat of anything that dieth of itself
This law is repeated from (Leviticus 17:15) , (See Gill on Leviticus 17:15):

you shall give it unto the stranger that is in your gates, that he may
eat it ;
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 (Leviticus 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:

or you may sell it unto an alien ;
an idolater, one that was neither a proselyte of righteousness nor of the gate, an entire alien from the commonwealth of Israel; one that was occasionally in the land of Canaan, or was travelling in it or through it, to such an one it might be sold:

for you are an holy people unto the Lord your God ;
separated from all others, and devoted to his service, and therefore must live on clean, food and good meat, and not eat what others might:

you shall not seethe a kid in its mother's milk ;
this is the third time this law is mentioned; refer to the notes, (See Gill on Exodus 23:19), (See Gill on Exodus 34:26); the reason of which repetition, the Jewish writers say ^F19 , is, that it is once said to forbid the eating it, a second time to forbid any use of it or profit by it, and a third time to forbid the boiling of it.

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FOOTNOTES:

• F19: Maimon. & Bartenora in Misn. Kiddushin, c. 2. sect. 9.