The storm is currently a Category 4, and forecasters expect it could grow to a Category 5 by the time it strikes Tonga Monday night.
Residents can expect soaking rains and winds that can exceed 200 kilometers per hour.
Authorities are already warning people to seek strong shelter and pick up anything they find on the ground that could become a deadly missile when the winds blow.
Gita struck Samoa and American Samoa Saturday, resulting in significant damage, including a lack of clean water in some areas. No deaths have been reported.