Building on Gentoo
From Gnash Project Wiki
Gnash is so far considered unstable on gentoo, so before you can install it, you need to unmask it. The normal way to do this is to add an entry to /etc/portage/package.keywords. For example
# The GNU flash player www-plugins/gnash
You probably want the browser plugin. If so, an entry in /etc/portage/package.use will do:
# Build the gnash browser plugin www-plugins/gnash nsplugin
After that, it should be sufficient to run
emerge gnash

