Here are some of the ways to run Ruby on Rails behind Apache.

Apache + mod_fcgid

Reference Howto Setup Apache With FastCGI And Ruby Bindings.

Apache mod_fcgid

Make sure mod_fcgid is available, if not, install it
# emerge -pv www-apache/mod_fcgid

Ruby fcgi

I was not able to install it via gem. See error below:

# gem18 install fcgi
Building native extensions.  This could take a while...
ERROR:  Error installing fcgi:
        ERROR: Failed to build gem native extension.

/usr/bin/ruby18 extconf.rb install fcgi
checking for fcgiapp.h... no
checking for fastcgi/fcgiapp.h... no
*** extconf.rb failed ***

It turned out that I didn't have fcgi lib installed. Actually, using emerge could satisfy the boken dependency

# emerge -pv emerge -pv dev-ruby/ruby-fcgi

Another option is to download Ruby fcgi from http://raa.ruby-lang.org/project/fcgi/ and install from source:

# tar xzf ruby-fcgi-0.8.7.tar.gz
# cd ruby-fcgi-0.8.7
# ruby install.rb config
# ruby install.rb setup
# ruby install.rb install

With this option, you also need the fcgi lib. 

Configure apache

There isn't anything we need to do with apache configuration, other than adding "-D FCGID" into APACHE2_OPTS variable in the file /etc/conf.d/apache. The restart apache.

In your project/public/.htaccess file, however, you need to:

1. Comment out the following lines:
   AddHandler fastcgi-script .fcgi
   AddHandler cgi-script .cgi
   RewriteRule ^(.*)$ dispatch.cgi [QSA,L]
2. Add a line:
   RewriteRule ^(.*)$ dispatch.fcgi [QSA,L]

Apache + mod_proxy_balancer + Mongrel

Here are some greate reference on how to set it up:

• Scaling Rails with Apache 2.2, mod_proxy_balancer and Mongrel
• Time For A Grown-Up Server: Rails, Mongrel, Apache, Capistrano and You

Apache + mod_rails

This is another option to check out. I think it is fairly new. http://modrails.com/. In portage, it is called "www-apache/passenger". "emerge -pv www-apache/passenger" will give you the info.