Reference documentation for Draw2d and GEF 3.x is provided in form of the GEF and Draw2d Plug-in Developer Guide, which can be found in the Eclipse Help. It contains a Programmer's Guide as well as the API Reference (Javadoc) for Draw2d and GEF. Reference documentation for Zest 1.x is currently not provided.
The following material provides additional documentation and may serve as the basis for building up (further) reference documentation.
There are also several Articles, Tutorials and Slides.