It sounds like its a tight press fit, and it will require a lot of force to remove it; if you don't have a gear puller then you will have to improvise. One, if you don't care if the spur gear gets demolished and if you can get access to a large bench vice, then you could probably just put it in there and "smoosh/squeeze" it off. If you don't have access to a vice then you should quickly heat up the spur gear (with a blowtorch for about 10 seconds), then quickly use a rod a little thinner than the shaft- brace the gear between something and give it a good whack with a hammer. I've included a picture description.