How do I keep my gelding from biting the lead rope while leading? It sounds like your boy is horse that likes to play a lot or he also could be a horse that has a little bit of confidence issues and so by holding on to the lead rope it acts like a pacifier to him and so he is avoiding the pull on his head by holding it. In any case it would be best to have him learn that chewing on the lead may not be all that great of a thing to do.
What I would try is discouraging him chewing on the lead and encourage him to not want to bite it. If he is a horse that likes treats observe him while you start to lead him. If you notice him attempting to grab it make a sudden flick with your wrist while holding the lead so that you cause a wave on the rope so that the snap bonks his chin. Then when he leads fine for a half a dozen steps go ahead and give him a treat. Practice this each time you lead him and try to make the distance between giving treats further and further apart.
If he already is holding the lead rope then go ahead and stop and take the lead with both hands, one and each side of his mouth, and try to slowly move it back and forth in his mouth. After a bit he'll get tired of holding it and want to spit it out. You then keep holding it in his mouth for another 15 to 30 seconds and then draw it out. Give him 15 to 30 seconds to think what happened and try leading him again. The key is to catch him before he grabs hold but you don't want to punish him if he already has a hold of it but just make it uncomfortable for him so that he will not want to keep holding it.