First, chmod index.php to 744, all other files in your Gallery installation to 644, and all folders to 755. Second, check to be sure that there are not any php_values specified in the .htaccess file that resides in your gallery's folder. Lastly, if you're still having troubles, check your Error Log in cPanel.
If you installed Gallery through Fantastico - maybe try an upgrade?
If you still have issues, you can contact us with a support ticket.