Since there were no Gentiles in the Great Congregation in the desert at the time of the Tabernacle, any NT interpretation of Gentiles in the Great Congregation would be erroneous interpretation, violate the OT, and break Scripture.
You do not know that there were no Gentiles who left Egypt with the Hebrews. There quite likely were, if you look even casually into ancient history. The Hebrews did intermarry at least in the case of Joseph.
Other things make it quite likely that not all who were in the desert at the time of the Tabernacle were Hebrew. From the web.
Around the time Joseph’s family migrated to the region of Goshen in the extreme north of Egypt, many other groups from Canaan and beyond were also migrating in, presumably due to the great famine which wracked the ancient world at the time. These in turn probably helped to pave the way (at least culturally) for the invasion of the Hyksos in the 18th or 17th centuries BC.
After the defeat of the Hyksos army, perhaps many Hyksos civilians would have been left behind, along with Canaanites and even Egyptian sympathizers. In the ancient world, this usually meant death, exile or slavery.
The Egyptians, eager to rebuild their kingdom and strengthen the land of Goshen at the northern approaches to the realm, probably turned on all of these groups with a vengeance, forcing them into a state of servitude along with the Hebrews.
So it is not unreasonable that these various slave groups became intermingled, with most ultimately adopting the customs and religious beliefs of the Hebrews. This would have swelled the ranks of the Israelites considerably. Within a few generations, they were probably all organized amongst the Twelve Tribes, indistinguishable in language or custom, and all worshipping the God of Abraham.
We have Ex 12:38-39 stating a mixed multitude went up with them And this from Ellicott's Commentary
(38) A mixed multitude went up also with them.—Nothing is told us of the component elements of this “mixed multitude.” We hear of them as “murmuring” in
Numbers 11:4, so that they seem to have remained with Israel. Some may have been Egyptians, impressed by the recent miracles; some foreigners held to servitude, like the Israelites, and glad to escape from their masters. It is noticeable that the Egyptian writers, in their perverted accounts of the Exodus, made a multitude of foreigners (Hyksôs) take part with the Hebrews.
So that shoots holes in your "God saves according to DNA!" theory.
