Overwrite and extend Gutenberg gallery block. Visit blog post https://bit.ly/34OMvdz

extend_gutenberg_gallery_block.css

.wp-block-gallery label {
  cursor: pointer;
}
.gallery-item-modal,
input[type="checkbox"] {
  display: none;
}
input[type="checkbox"]:checked + .gallery-item-modal {
  display: flex;
}
.gallery-item-modal {
  align-items: center;
  background-color: rgba(0, 0, 0, .8);
  bottom: 0;
  justify-content: center;
  left: 0;
  position: fixed;
  right: 0;
  top: 0;
  z-index: 100000;
}