Warning after activation plugin

Hello,

I want to let you know that when I installed and activated this plugin I got to see the followin:

Warning: Missing argument 1 for plugin_dir_url(), called in /public/sites/burogu.avo-forum.nl/wp-content/plugins/gamma-gallery/gammagallery.php on line 13 and defined in /public/sites/burogu.avo-forum.nl/wp-includes/plugin.php on line 599

Warning: session_start() [function.session-start]: Cannot send session cache limiter – headers already sent (output started at /public/sites/burogu.avo-forum.nl/wp-includes/plugin.php:599) in /public/sites/burogu.avo-forum.nl/wp-content/plugins/si-contact-form/si-contact-form.php on line 1997

The plugin generated 274 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.

I use the theme Esplanade.

http://wordpress.org/extend/plugins/gamma-gallery/