The gear RXG0009 was used in 90’s Technics/Panasonic drive mechanics. It is part or mechanism that is driving tape transport. Because it was made from semi-flexible material it tends to crack during the time.
Faulty gear can be replaced with 3D printed one. This type of drive mechanism is quite easy to disassembly – just follow service guide steps. You can find the gear here in schematics: