Bearings are what allow the gears to rotate efficiently. They are generally built out of bronze or brass, and they suit into races (or grooves) while in the gearbox housing. Synchro rings have enamel on them that mesh with the enamel about the gears, and so they also have friction https://filtergearbox19741.blogsidea.com/40511563/filter-gearbox-an-overview