The gears are put on HOT, and then when they cool they lock into position. A good puller usually can pull them off, but you may have to heat them with a torch if they are stubborn. don"t heat the shaft, just the gears.
Depending on its age, it might be a 2 piece gear, or a single gear. I"d use a bearing puller or something behind the gears so your puller jaws aren"t pulling on the gear teeth (they break).
IF you go here, you will find gravely manuals that will show you how to do all this stuff.