The nut is right hand thread and normally locked with a tab washer. If you're clever, you may be able to pull off the gear with screwdrivers, brake adjuster spoons or the like but it probably won't move unless you have equal and opposite forces on either side of the gear. You can make a puller if you have access to a lathe. I found a battery terminal puller, the more expensive type, worked well for me. The gear of course is keyed to the shaft and is a light press fit. You may be tempted to remove the locating screw for the timing side bearing bush and withdraw the whole assembly to be dealt with later, but I wouldn't recommend that. The case itself is quite thin in that area and is always looking for an excuse to crack when an unusual force occurs. You don't want that - trust me.