How do I create a gallery post https://colorlib.com/activello/post-format-gallery/? DO I have to use a child theme? If so how do I do that?
I hope you are well today and thank you for your question.
You can try achieving this by using the Jetpack plugin Jetpack – WP Security, Backup, Speed, & Growth – WordPress plugin | WordPress.org and its gallery module as described here Tiled Gallery Block
You are most welcome here