@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."
Which would make it a kind of drasha, like the Yerushalmi stated.
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
Edit: Side-note: I have been thinking about this story again this week and I would like to suggest a different opinion, which is that the first "quote" from the sectarian wasn't an actual quote, nor was it a drasha as the Yerushalmi put it. Rather, he made it up. For evidence, we can see that R"G did not directly quote from the Torah when he said "אֲמַר לֵיהּ, כְּתִיב לַן: בִּמְקוֹם בְּרָא, בְּרַתָּא לָא תֵּירוֹת" - he was merely paraphrasing from "אִישׁ כִּי יָמוּת וּבֵן אֵין לוֹ וְהַעֲבַרְתֶּם אֶת נַחֲלָתוֹ לְבִתּוֹ" (Bamidbar 27:8). However, the sectarian still mimicked R"G's wording when passing judgement, in both cases. And since he ruled differently depending who bribed him, I would say that he invented this "quote" as a basis for his favoring first Imma Shalom and later R"G. This understanding is different than the way Herford, at least, read the text (in terms of who said what). I don't know why he thought he would get away with it, although it is noticeable that he is unaware who the Nasi, R"G, and his sister are - so it seems that in his mind, he could get away with anything here.
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.