Help/HowTo here.
Also, we are starting a new wiki in https://bcscle.atlassian.net/wiki/spaces/BWW/overview.
wp-content/plugins/gallery-lightbox-slider/js/jquery/photobox/jquery.photobox.js
in
imageLinksFilter : function(obj)
//Start Update Soumen Mar 5 2020
//caption.content = ( thumbImg.getAttribute('alt') || thumbImg.getAttribute('title') || '');
caption.content = ( thumbImg.getAttribute('alt') || thumbImg.getAttribute('title') || thumbImg.offsetParent.innerText || '');
//End Update Soumen Mar 5 2020
Above code will show image caption in lightbox.
Details here.