To be able be to use your webcam via mplayer on Gentoo you need to compile it with the v4l USE flag, for that you need to add the USE flag to the package.use:

# webcam mplayer
>=media-video/mplayer-1.3.0-r6 v4l

If mplayer is already installed you need to re-emerge it using with that new flag:

sudo emerge --changed-use -a media-video/mplayer

Now you can test your webcam:

mplayer tv:// -tv driver=v4l2:width=640:height=480:device=/dev/video0