Diana 60 Break barrel stop cocking

My first guess would be a tooth has broken off of one of the cog wheels and the broken piece is jamming the function of the giss system. Remove the caps one side at a time and check for missing teeth. If you remove both caps at the same time the alignment may shift slightly, making them harder to reinstall.

Stop, do not touch rig. When was it last rebuilt? 

Wadcutter's idea seems correct to me. 
If you are not experienced in these rigs "I" would ship it off for repair - I do work on my own pre-charged and springers biut not a Giss system-

David Slade at airgun werks has parts & can rebuild.

But, if you are doing it could you please take & post pictures.

Not your rig but something for newbies to look at to get some idea:
http://anotherairgunblog.blogspot.com/search/label/diana%2010

John

This idler cog was out of a Diana 75 I rebuilt. Never found the tooth, but the rifle had a stiff spot while cocking.


If you decide to tear into it, here is an album of photos from my Diana 60 tear down. May come in handy.
https://flic.kr/s/aHskQM3fqz