There is a great set of instructions on Wojciech Bednarski's blog. any additional steps are documented here.
First install MySQL and PHP.
Next, emerge wordpress using
sudo emerge wordpress
Then edit wp-config-sample.php in the wordpress installation directory (e.g., /var/www/localhost/htdocs/wordpress/) using the following values
// ** MySQL settings ** //
define('DB_NAME', 'wordpress'); // The name of the database
define('DB_USER', 'username'); // Your MySQL username - e.g. joebloggs,
define('DB_PASSWORD', 'password'); // ...and password
define('DB_HOST', 'localhost'); // your hostname, e.g. example.com
and save it to wp-config.php in your wordpress installation directory
When the installation link (e.g., http://localhost/wordpress/wp-admin/install.php//) is loaded the following message appears: “Your PHP installation appears to be missing the MySQL which is required for WordPress”. This means there was a problem with the PHP installation, and the mysql use flag was not set. Therefore you must add mysql to the use flags in /etc/make.conf and reinstall PHP. Once this is done, you can reloaded the installation link and began setting up wordpress.