Just to clarify, I’m using the latest versions of WordPress (5.6.1) and a bare-bone Child Theme with Astra 3.02, and I just tried Chrome (Google Chrome is up to date, Version 88.0.4324.150 (Official Build) (x86_64)) I cannot get Responsive Lightbox to let me insert a gallery, whether I use the WordPress Image Gallery block or the Responsive Lightbox Image Gallery block.