Is there an error in the statement "So Gabriel revealed to His servant Muhammad" mentioned in Tafsir al-Tabari regarding the verse, knowing that the Prophet Muhammad, may Allah’s prayers and peace be upon him, is the servant of Allah the Almighty alone?

There is no error in the statement; the pronoun in the verse and Al-Tabari's words refers to Allah Almighty. It is common in the Qur’an and Arab speech for a pronoun to refer to something not explicitly mentioned if it is known to the listener, such as the pronoun in "عليها" (on it) referring to the earth in Allah's verse: "And if Allah were to punish the people for their wrongdoing, He would not leave upon it any creature." [Fatir: 45]. Likewise, the pronoun in "به" (with it) referring to the place in Allah's verses: "And stir up thereby [i.e., with their hooves] dust, Then penetrate thereby [i.e., in the center of the enemy] a mass [of soldiers]." [Al-Adiyat: 4-5]. Al-Alusi, Al-Nasafi, and Al-Shanqiti have mentioned various examples of a pronoun referring to an unmentioned antecedent that is indicated by the context or situation in the Qur’an and Arabic poetry.