Might be worth while to contact Skywatcher USA directly. I think there might be some more mounting hardware missing than just the gear so they are the best bet for having the parts you need.
I'm not an expert planetary imager so if I say anything wrong please let someone correct me. I think it's a workable setup but as it stands but it's not ideal. I think the EQ 3-2 is undermounted for the size and weight of a 150mm Newtonian. Also the newtonian is F5 with a focal length of 750mm and that's less power than what you'd want to be operating at for planets. Image scale would be very small. As it stands for a 3.75nm pixel camera you are at about 1"/ pixel and you could do better to push it up to F15 (~0.33"/pixel) with a 3x barlow. Personal thought is to go with a longer focal length Mak or SCT. However your 150 newt would perform very well for DSO photography later on if you upgrade the mount.
I don't own this mount so I can't say for sure. It looks like there is a small brass gear on the right side of the "head". I would think this engages with a large, fixed gear that would be mounted to the base (missing in your picture). That way when the small drive gear starts moving it rotates the AZ axis with a significant gear reduction. This is probably the mechanism that sits in the "bowl" section in Gfamily's picture.
Hello everyone, My name is Daniel and it's nice to join you all here. I've been doing visual astronomy on and off for about 12 years now and started out with a 6" dob which taught me lots about the sky and how to use my equipment. I dabbled with webcam imaging on it as well and started to appreciate the meaning of "patience" when it comes to keeping an image on a tiny CMOS with a manual mount. I took a 6 year break from serious astronomy time because of school and work demands putting a premium on sleep. I downgraded to a C90 at that time for quick grab and go and it stays with me today. I'm now getting back into the swing of things again. I now have a SW ED80 along with the C90 on an EQ modded AZ-GTI mount. I've always liked photography and have set myself up to do this properly this time. It's a steep learning curve but all that reading when I couldn't actually go out gave me a bit of a foundation to work with. I've been active on CN as well and wanted to say hello to everyone here too. Hope to dive back in with a great group of people! Clear skies, Daniel