The uCR distribution includes patches against GDB 4.16 that allow one to debug threads in a uCR application. For supported targets, it also includes the proxy code needed in the application to support debugging with GDB.
The GDB support is intended to run in its own thread, communicating through some device with the actual debugger running on the programmer's machine. The application must explicitly include the GDB interface code, but may then mostly forget it.
With the interface code installed, the debugger has access to all the threads in a uCR application, as well as all the memory accessible to the processor. The GDB interface runs as an ordinary thread so it is naturally vulnerable to the evil designs of particularly nasty bugs, especially those that trash memory and leave the system unusable.