Daf Yomi - Beitza 24 - Halacha is not like Rabban Gamliel

The Gemora explains the Mishna as follows: An object which there is a doubt if it was sufficiently prepared from before Yom Tov is prohibited. Rabban Gamliel disagrees and permits them. The Mishna relates that there was an instance where a Gentile brought fish to Rabban Gamliel on Yom Tov and he stated that the fish are permitted but he did not wish to accept the gift from the Gentile. Rashi states that the Gentile was an enemy of Rabban Gamliel. Rav Yehuda rules in the name of Shmuel that the halacha is not in accordance with Rabban Gamliel.

Why did the Gemora have to explicitly rule against Rabban Gamliel, isn't the rule that when an individual argues with the Chachamim, the halacha is always according to the Chachamim?

Pnei Yehoshua and Reshash both answer that perhaps one would have thought that since Rabban Gamliel ruled for everyone according to his opinion, it should be considered a 'maaseh Rav' and the halacha should be according to Rabban Gamliel. The Gemora concludes that this is not the case and the halacha is in accordance with the Chachamim.

Rav Menachem Kohn Zt"l in his sefer Ateres Avi explains based on this as to why the Gemora states the halacha is not in accordance with Rabban Gamliel. It should have said, the halacha is in accordance with the Chachamim. Since the novelty is that we do not rule in accordance with Rabban Gamliel even though it was a 'maaseh Rav', the Gemora specifically states that the halacha is not according to him.

I am curious as to what the rules of 'maaseh Rav' are and if they would apply in this case when Rabban Gamliel did not actually eat from the fish. If anyone should have any information on this, please let us know - thanks.

Look at the Sefer Ateres Avi where he cites another explanation from Rabbi Mordechai Nadoff as to why the Gemora states the halacha is not in accordance with Rabban Gamliel and not that the halacha is in accordance with the Chachamim.