Implementing a modal(lightbox) for all images in all post-gallery posts

I’m trying to make a photo gallery page for all my photos in my post-gallery post and have a modal open when you click on the photos. In the page there will probably be almost 100 photos and i’m not sure how to do this in a good way without duplicating the code for the modal 100x. I have considered using plugins but i’d doing this as part of making my own website and learning developing wordpress themes so i’d prefer if the solution is done with code.

How to extend Media Library (WP 4.4)

I am trying to implement a new tab next to Media Library. I couldn’t find solution to implement this. I found an example how to implement a new tab in this link: https://gist.github.com/Fab1en/4586865 However the tab doesn’t show up in the modal form triggered by Add Media button. But if I call the media popup for Featured Image or any other custom button the “New Tab” I created shows up. I am really lost here, not sure why those tabs show up in one place and don’t in another.