I don’t really know if they are essential, but if the church has bells, then they should be rung at this time according to local custom. I don’t think it really matters how many bells are rung; my parish has fake bells, and the machine is shut off all of Lent and then turns on for the Gloria at the Evening Mass of the Lord’s Supper on Holy Thursday, turns off, and then comes back on for the Gloria during the Easter Vigil. We also ring the altar bells during both Glorias.