At the beginning of your module you must initialize the RSG2 library.  This is done with the following code:

// initialize RSG2
global $mosConfig_absolute_path;
require_once( $mosConfig_absolute_path.'/administrator/components/com_rsgallery2/init.rsgallery2.php' );

Notice the use of require_once().  The library only needs to be initialized once.  After that you can access any of it's functions or variables anywhere.

For example:

$gallery = rsgGalleryManager::get( $parent );

Or:

global $rsgConfig;
$rsgConfig->get('debug');

Notice that we had to declare $rsgConfig as global before using it.  Because of require_once() init.rsgallery2.php may have already been included outside the scope of your code.  This means you must declare you want to use the global RSG2 variables before using them.