I'd need to see a good argument for why galleries need to be sortable, and what they would be sorted by.

The only orders I can think of are the order the gallery owner wants, and the title order. That's not a lot of scope for sorting, and galleries aren't really title-based anyway, they're generally browsed visually.

This isn't related to catalogues though. Downloads and Galleries are not catalogues, catalogues don't provide any kind of special functionality and are their own thing.

Actually I do see sort options on galleries, on both flow mode and non-flow mode.