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.