I've recently committed a bunch of bug fixes for GDB on S/390 Linux to the public sources (see http://sources.redhat.com/gdb). The GDB test suite results suggest that these are a big improvement over the GDB's currently included in the big distributions. Of course, the test suite results don't always predict the experiences of real users, so I can't say whether it'll be any better in real life.
If you use GDB a lot, and are comfortable rebuilding it from the sources (it's `configure; make', like any other GNU package), you might give it a try. I can't provide any assistance with compilation, installation, or usage, and my time will soon be officially allocated elsewhere, but I'm quite interested in bug reports --- info that I can use to make GDB better.
