The Gottlieb factory backdoors should lower the volume on the sound of the bells once installed back on the head of the game.

Make sure the 0-9 one point bell unit striker arm is striking the bell correctly. Sometime it looks like the striker arm is close enough but in reality it's not position close enough to the bell.

Also check the 10 point bell unit while your at it. Sometime the metal plunger plastic tip isn't attached to the metal plunger or it's not adjusted properly.
I do not think there is a 100 point bell in a GiGi?