Yes, extending the debugger is more work than extending the inspector. However it depends on what you want to do. Do you need to change the UI of the debugger or just add new actions?

To create a new UI you need to subclass GTSUnitDebugger and override debuggerStructureIn: and debuggerTransmissionsIn:.  You can the extend the UI using glamour presentations. On the class side of your UI you need to implement #defaultTitle, #handlesContext:. If you need to customize it more you can override at #helpClass, #sessionClass, #variablesBrowserClass and #availableAutomatically.

To create new actions you need to create subclasses of DebugAction. There are a lot of examples in the image. To link a action with a UI widget you need to add to the class side a method that just has an annotation. Different widgets load actions that have various annotations. For example the stack widget loads actions annotated with #gtStackDebuggingAction.

Let me know if you need more information to get started. Unfortunately there isn't a guide that you can use.

Cheers,

Andrei