A documentation how to install a PECL-Extension is available in the PHP-Manual http://www.php.net/manual/en/install.pecl.php

The extension documentation is now available in the PHP-Manual: http://php.net/manual/en/book.gnupg.php

The extension supports only gnupg version 1 and the version 2 is not currently supported. It should be fixed in the extension version 1.5.