Is it permissible to refrain from performing ghusl (ritual bath) with water for five months, including the month of Ramadan, based on the advice of a specialized doctor after undergoing a surgical operation, or is ghusl obligatory, along with prayer and fasting Ramadan?

If there is an excuse that prevents ghusl, then tayammum for janabah is permissible. However, it is preferable to consult another specialist and trustworthy doctor, especially if they are Muslim, to confirm the condition. If the doctor who informed you of the danger is the specialist and you trust them, then there is no harm in accepting their word and performing tayammum until the specified period ends, in order to preserve your health and safety.