No, you are wrong. They would not be gentiles and they would not be Jews. They would be "yisrael".
A gentile is someone who is not of one of the twelve tribes of Israel, therefore the lost tribes are not gentile.
Member "Lost Tribe" said that, "from a Jewish perspective," the lost tribes would be gentile. This is false. "Lost Tribe" knows nothing about what the Jewish perspective is. He knows nothing about Jewish law on this subject, nor does he care to hear about it.
I keep looking for the "beef" in your argument and finally found a few crumbs from a stale hamburger.
Member "Lost Tribe" said that, "from a Jewish perspective," the lost tribes would be gentile.
Well, that just means they would "not quite like Jews", because he defined it that way. No other definition, yours or anyone elses is applicable.
You are comparing apples and oranges. If you read his post about "gentiles" carefully you'll see he is not claiming to be a Jew or think like a Jew or have some unique "Jewish perspective" or sensitivities, real or imaginary. He might have worded it better, but he defines what he means by the phrase before he uses it, and that definition is different from yours.
Lost Tribe says, Gentile means "not quite like us". That has nothing to do with theology or even being Jewish. It just means "not quite like us". So he uses gentile from a "Mormon perspective" (or reference point) as an example; Mormon gentiles are not quite like Mormons.
Then he uses gentile from a "Jewish perspective" (or reference point) as an example; Jewish gentiles are not quite like Jews. This has nothing to do with what you keep squawking about!
The conclusion is, THERE IS NO CAUSE FOR YOUR BEEF. Forgettaboutit.