ZEST was included in the GEF library. I am a bit confused about when to use GEF and when to use ZEST. I would like to know when using GEF is better than using ZEST, and the other way around?
Simply put, GEF is better suited for Graphical Editors. For example, if you need to let the user draw a flow chart. While Zest is better for graph output. Zest still enables interaction with graph components (like selection and movement).
If you need to draw a graph (nodes and arcs), Zest is a better choice because it was specifically designed to this task. It contains exactly the components you need, including graph layout algorithms. From my experience, it is very easy to use (it was used in nWire).