Installing on Gentoo

Thunar will be installed when you emerge xfce4:

# emerge -avt xfce4

If you run some other desktop environment or window manager you can try installing just Thunar with emerge -avt thunar. This will pull in some Xfce4 dependencies, as Thunar is designed for Xfce.

You may want to read the Gentoo Xfce Configuration Guide to learn how to properly install and configure Xfce and Thunar.