It doesn't seem likely the "gag" item would be a ball. I think of a ball being round, and this item was described in the People mag article as being oval, 2 inches by 3 inches.
The gag was described as "a large yellow oval plastic device," 3 inches long and 2 inches wide, according to the report.
​http://www.people.com/article/cause-of-death-sierah-joughin-ohio-missing-girl
As it appeared that her front left tooth was chipped, I question whether such was from the helmet or the item that is being discussed. :dunno:
I was surprised a few days ago to learn that the helmet was on the other side of the road from where her bicycle was found, iirc.
ETA: Adding link of reference above
"The report further detailed that Sierah’s body was found hog-tied with ropes and duct tape and her hands were handcuffed. In addition, it appears she chipped her left front tooth while she was gagged." http://www.wtol.com/story/32728835/autopsy-results-show-sierah-joughin-died-of-asphyxiation