@moses in this answer refers to Edgar J. Goodspeed, who in "The Possible Aramaic Gospel", pg. 335, brought Hermann Strack's suggestion that:
"It is probably, as Strack suggested, a projection of Paul's teaching in Gal. 3:28: "There is no room for 'male' and 'female'; for in union with [CJ] you are all one." This would mean that the term "Gospel" is used loosely of Christian teaching in general, whether through Matthew or Paul."
This suggestion is also mentioned by Dr. Yonatan Feintuch in his essay "סיפור ר' יהודה נשיאה וירושת הבת - הלכה, משפט וספרות" (The Story of Rabbi Yehudah Nesi'ah and the Inheritance of the Daughter - Halacha, Law and Literature), pg. 214, footnote 39, but he thinks it's too far-fetched.
Robert Travers Herford in "Christianity in the Talmud and Midrash" suggests that it might have come from a Christian book that is no longer extant, as the verse that Rabban Gamliel quotes may be the same as the one in Matthew, but there it's in the beginning, while RG tells the Christian to look at the end of the Evangelion.
David Tzvi Müller in "Zum Erbrecht der Töchter" (The Daughter's Right of Inheritance) mentions that Joseph Halévy saw parallels between this quote and the writings of Philo1. Müller himself, however, wrote:
"As you can see from this passage, it is not a Sadducee law, but apostolic law, because Jesus hardly ever uttered the sentence that son and daughter inherit together. In my opinion it is questionable at all whether an episode is really being told here in the Talmud and not rather a satirical anecdote."2
1 Halévy bases this on a book called Philonea, which brings Philo in Greek. The best Google Translate could do is a very partial translation and I could see that the text does discuss inheritance and sons and daughters, but I'm not sure from where in Philo's writings this was taken from.
2 Translation from German via Google Translate.