To add a Gallery using Catch Gallery, Edit a Page => Add Block => Formatting => Classic. You will find a list of options now. Now, from the list of buttons click on Add Media. Then Select some images and Create a New Gallery. Then you will find Gallery Settings on the right hand side with Size and Type Options and finally Insert Gallery. From the type drop down select the type of gallery you would like to display.

Let me know if you are still having issues using Catch Gallery.