You can compile it to use gstreamer instead of ffmpeg if you want. Anyway, gnash works fine as a standalone player at least, but it's slower than the official player and the stuff I tried that doesn't work in official flash didn't work in gnash either. So from my experience there are no real benefits right now. I built against the latest from the git repository.

There seem to a fair amount of activity on gnash at least so perhaps it will be better in the future.