It is most probably the compression washer that has disintegrated into dust, a common fault on these with the Giss system. They are not the easiest guns to work on and a spring compressor is highly recommended as there is a lot of pre-load on the three spring system. Make sure that you mark the position of the pinions so that they go back in exactly the same position so that the two piston timing is correct.
Neil
Current airguns:- Steyr LG110: Steyr LP10: Air Arms HFT500: Weihrauch97 fully customised.