Gantin is a peer, another bishop. He has no standing to excommunicate an archbishop. Your posting this proves nothing.
Your posting this proves something - your ignorance of the structure of the Church.
According to ultima, Roman Congregations have no authority. What an amazing new discovery - a novelty in the true sense of the word.
In exercising supreme, full, and immediate power in the universal Church, the Roman pontiff makes use of the departments of the Roman Curia which, therefore, perform their duties in his name and with his authority for the good of the churches and in the service of the sacred pastors. (Vatican II, Christus Dominus 9)
And if the Council won't do:
They argue that the sentence of schism and excommunication pronounced against them by the Archbishop of Tyana, the Apostolic Delegate in Constantinople, was unjust, and consequently void of strength and influence. (Bl. Pius IX, Quartus Supra)
According to ultima, the "neo-schismatics" were right and Bl. Pius IX wrong - the excommunication by the "peer" of the schismatics - the Archbishop of Tyana - was in fact "unjust, and consequently void of strength and influence".
The Roman Curia is the ensemble of the dicasteries and bodies that assist the Pope in the exercise of his supreme pastoral office for the good and in service of the Church throughout the world and of the particular Churches, assisting in the maintenance of the unity of the Faith and the communion of the People of God and in the promotion of the proper mission of the Church in the world.
There are a number of departments of the Roman Curia, each with their own responsibilities and competencies. The most important parts of the Curia are the Secretariat of State and the various Congregations. Then, there are Pontifical Councils and Pontifical Commissions.
Apart from the formal structures of the Roman Curia, the Pope may convoke an extraordinary consistory of the College of Cardinals to consider special difficulties or questions. In this way, they can advise him as a body, not just the Cardinals who advise the Holy Father as part of the Roman Curia itself, but all the Cardinals from around the world. The Pope has done this on a number of occasions during his pontificate. This is quite a novelty and was not done often during the pontificates of any of his modern predecessors.
In particular, the Pope has established a "Commission of Cardinals for the study of the organizational and economic problems of the Holy See" which meets regularly to assist the Holy Father with the economic management of the Vatican. The Holy Father established this commission early in his pontificate to help turn around a growing deficit in the Vatican's books that he inherited from his predecessors.
The Congregation for Bishops is responsible for the delineation, division, establishment and merging of Dioceses, for the consideration of candidates to be appointed to Dioceses and the recommendation of candidates to the Holy Father. It is also responsible in a more general sense for assisting the Bishops in their governance of their own Dioceses. It receives their quinquennial reports and organizes their visits "ad limina Apostolorum".