festival-gaim will NOT work unless mbrola is installed. ebuild needs to be changed so that mbrola installs before festival-gaim.
I just checked this out- the mbrola voices are optional in festival-gaim. Just set the plugin to use the voices provided by festival, and festival-gaim will work fine. Festival-gaim can, however, use the mbrola voices if you do chose to install them, however this is up to the user to chose which voices he/she wants to install.