Leo,
I was wrong. I verified that hardware breakpoints are working for the Cortex M3. The GDB keyword for setting hardware breakponts is 'hbreak'.
You should be able to do any type of basic stop-mode debug through the GDB agent. It supports the RSP protocol primitives that required by GDB. The task-aware debug support in the GDB RSP protocol is not implemented in the GDB agent however, meaning the agent itself does not support Linux application debug. Of course if you are doing Linux application debug you probably would be running a GDB server anyhow.