@mki: Hello there,
Looks like you didn’t follow the instructions well.
To insert catch gallery in Gutenberg editor
Open the page/post where you want to create Gallery, click on Plus Icon Plus Icon and “Classic” to add the classic block.
Click on “Add Media” Icon Add Media Icon.
Click on the “Create Gallery” button and if you need to upload new images then you can do it here from the “Upload Files” tab or if you want to use existing images in your site then you can select it from “Media Library” tab.
After selecting images for the Gallery, you need to click on the “Create a new gallery” button.
After that, you will see Gallery settings on the right panel of the “Edit Gallery” screen, there you can select the “Columns”, “Size” and “Type” depending on the style you want.
Finally, click on the “Insert gallery” button, publish or update your page/post.
Let me know if you are still having issue setting up catch gallery.