I don't know gtk-gnutella, does it depends on mplayer??? becasue the error i see makes sense, it is just saying that it cannot download mplayer, which is normal cause you cancelled the repository... However, if you tried to install mplayer *(or packages depending on it) before it is maybe becasue there is something in the cache... try apt-get clean and let me know how it goes... mmm.. .confusing
EDIT: and what is this mozilla-mplayer thing? you have it installed?