It depends on the time frame involved, and it is generally accepted that initially most of the gladiators that competed were slaves by virtue of being prisoners of war, criminals, or slaves bought for the proposes of being sent a gladiatorial school. However, it is estimated that by the end of the Republic that about half of the gladiators were free men that volunteered, primary for the money and glory involved.
Thus, while the claim might not be entirely accurate, it is possible that at some point there were more volunteers than not; however, over the the history of the gladiatorial games, the volunteers were in the minority.