Tools
Piranha Bytes did not release a modkit for their Genome engine, but the modding community has released a wide range of tools to work with the game's files and the engine itself.
Info
This page is under construction, for now, only handful of links are present.
Gothic 3 SDK
Georgeto, inspired by NicoDE's Risen SDK, has created an SDK for Gothic 3. It can be used to manipulate the engine in the similar way Union is able to manipulate ZenGin.
GitHub repository