If you want to be able to access your Gallery2 through the server's URL, you have to edit the main configuration file, which is config.php and it is located into the main Gallery2 directory. You can edit the file using the File Manager in your cPanel. Once you open the file in edit mode, you have to replace the following line:

$gallery->setConfig('galleryBaseUrl', '');

with this one:

$gallery->setConfig('galleryBaseUrl', 'servername.com/~username/gallery2/');

where "yourserver.com" has to be replaced with the hostname of your server, and "gallery" - with the name of your gallery's root directory.