When transitioning from piano to organ, you first need to learn how to pedal. That is fundamental, especially in gospel. If you are particular about learning how to pedal correctly, then you will appreciate it later.
After that, I learned everything from watching other organists and experimenting myself. The Hammond B3/C3 organs are very well-built instruments can take a serious beating on the keys and pedals. Getting the right growls, pops, whistles, etc. is all through trial and error. Try sliding up and down the keys, tapping them very quickly and making good use of the expression (volume) pedal while doing such techniques.
A lot times, when I need growling bass, I use pedals and my left hand to play the bass line, but with the left hand, I slide up and down the lower keys a lot to get that deep growl. When you hit several bassy keys on an 888800000 setting, you get growl.