Very sorry to hear about your fall. A gym fall is especially hard, because the only risks indoors are subjective (I think). I had a similar ride but luckily was caught inches short of the floor. I believe the problem comes from inappropriate use of the gris-gris. It is a very good belay device, but not foolproof as some would assume. Many people use the handle as a way to control the run of the rope, which I consider unsafe. The handle is meant to be released in the event of a fall, catching the rope in the cam inside, by pinching it. There will be some run before the rope is caught, but it works. Nevertheless is the belayer is using the handle as a way of controlling rope run, and accident can happen. The best way to use the gris-gris is to open the handle with one hand and use the other hand as a break hand, in the same way one uses an atc, as a passive belay device. This is made even easier if one uses the offset biner that is attached to the floor anchor, which the belay side of the rope can be clipped through, providing more friction, and then belay from the other side of that. The gym staffs always try to stress proper use, but often an experienced climber may not have checked into how the mechanism should be used, and may be lead checked, etc., without having really examined how the device actually works. I think every climber is inherently obligated to examine all working gear and understand it and we shouldn't overlook that obligation even when someone else has set up a belay system for us. A person who doesn't use the gris-gris should open it up, examine the system and understand how it works. This may have not been what caused your accident, but it's what happened to me.