The reason is: Gwyneth Paltrow is not Jewish! Her mother is a White Anglo Saxon Protestant, and by law and common sense, she clearly favors her Gentile mother, not her Jewish father.

That is why she will play the gentile. If you look at the father Bruce Paltrow, it's a totally different phenotype, and Gwyneth takes after her famous WASP mother! Common Sense!
Just like asking why Lisa Bonet played a role in a black family, even though she is Jewish also, and looks half white. But noone is wondering why she is cast as a black girl, instead of a white girl!