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.

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

g3dit

g3dit is a world data editor for Gothic 3 that allows you to view and edit .lrentdat, .node, .tple, and several other file types used by Gothic 3.
GitHub repository
World of Gothic release thread

G3IQ

G3IQ, the Gothic 3 info and quest editor, makes it possible to edit info and quest files, as well as the stringtrable.ini.
World of Gothic download

g3blend

g3blend is a Blender addon to import and export Gothic 3 actors (.xact) and animations (.xmot).
GitHub repository