I know that DSTREAM may work best with DS-5 studio. But out of curiosity, will this debug/trace unit work with some other (maybe free?) combinations of tools like GDB + OpenOCD? On its documentation it says DSTREAM "supports" 3rd party debuggers, but what does this mean?
DSTREAM has an API called RDDI that would allow other debuggers to use DSTREAM as it's communication channel to a target. I don't however believe there are any non-ARM debuggers that publically use this.
You can use DSTREAM with DS-5 Debugger, or the older RVD debugger (v4.0 or later).