Gentoo uses /etc/apache2/httpd.conf now, but you don't need to edit it to get php to work.
What you need to do is edit /etc/conf.d/apache2 and add "-D PHPN" where 'N' is either 4 or 5 depending on your php version like so:
Code:
APACHE2_OPTS="-D PHP4"
Now restart apache.