[Task]: Gamma node rules documented in Overleaf

[ThijsJenneskens]

derive the rules for the Gamma node in the Overleaf file

[wouterwln]

Btw @ThijsJenneskens I prompted Gemini 3.0 about rules and it spit out a perfect derivation for the backwards rule towards the shape so should not take more than 30 minutes to check the derivation and have everything documented.