You can remove title and description from image in admin -> Gmedia Library
If you want to keep them – the only way to hide them on frontend is to edit /wp-content/grand-media/module/phantom/js/jquery.gmPhantom.js file and replace 604-605 rows with this or remove them at all:
// $('.gmPhantom_CaptionTitle',this).animate({'height': 'show'}, LightboxTextDisplayTime);
// $('.gmPhantom_CaptionTextContainer',this).animate({'height': 'show'}, LightboxTextDisplayTime);
Do the same with 627-628 rows (to hide captions on mobile devices):
// $('.gmPhantom_CaptionTitle',this).animate({'height': 'toggle'}, LightboxTextDisplayTime);
// $('.gmPhantom_CaptionTextContainer',this).animate({'height': 'toggle'}, LightboxTextDisplayTime);