First a disclaimer.....not an expert on Gipiemme but have done a fair bit of wrenching. Anyhow the top tips are:
a) Be prepared for bruised and skin torn knuckles
b) Never underestimate the Japanese, they are very clever
c) You will need about four hands. All of them right.
d) Strangely, heavy percussion instruments don't work well on rear mechs.
Here goes:
1) Pop the spring in the mech body, locate it with the hole.
2) There should be a hole on the main steel bolt for the spring too. Again locate the hole to mate with the spring.
3) With the allen key in the main steel bolt, turn to get gradual tension. Difficult to say how much, start with
half a turn say. You want the spring to "compress" so I believe you should be turning in a clockwise direction.
4) The metal plate should slip over the main bolt; located by the flats. Looking at the wear marks, the side to
"us" in the picture is the side that mates with the main bolt. Looks like it is held on by force alone rather than
a circlip.
5) You can then try it. If you need more tension, put the allen key in the main steel bolt, dislodge the plate,
tension up another half turn, pop the plate back on.
6) I guess the other bolt is to affix the mech to the frame.
Best of luck, and hope I'm not stating the obvious and it's clear!