By default, the Gutenberg Gallery Block outputs full size images in 3 columns. This is excessive, especially on mobile, so I would like to set it in the theme or function file to output thumbnails. How do I do this? Changing the image size option in the Gallery settings seems to have no effect.
(I suspect this is because my homemade theme doesn't support these options correctly, but I cannot find any documentation on theming for new Gutenberg blocks).