Why would reverberation be a problem when using a gym for a concert?

Because multiple sounds will be heard over and over again thereby leading to noise and even confusion

Explanation: The reflection of sound waves from a hard surface is called echo and the multiple occurrence of echo is called reverberation.

During concert, several sound waves are released from the source (speaker). However, if the concert takes place in a gym, reverberation will occur.

A gym is a made up of a large space with little or no sound absorbing surfaces (such as sofas and carpets) thereby giving room for reverberation, that is sound waves will be frequently reflected until the sound wave exhausts it's energy. This will result in noise as the same sound will be heard repeatedly and will further lead to imbalance and confusion.