This is simply false. There is no record that Pope Gelasius ever decreed that anyone who taught the Assumption is a heretic.
Mary's Assumption was not official Church dogma until 1950. Why did it take over 1900 years for the "infallibility" to settle in? And why did it become dogma based on writings not believed by Church Fathers that lived closer to the actual events?
"Juvenal, Bishop of Jerusalem at the Council of Chalcedon (451 AD) made known to the Emperor Marcian and [his Empress] Pulcharia, who wished to possess the body of the Mother of God, that Mary died in the presence of all the Apostles and that her tomb, when opened upon the request of St. Thomas, was found empty; wherefrom the Apostles concluded that the body was taken up to Heaven." (Homily on the Dormition, PG 96)"
-A8