I’m using this, and it works for captions under images and the individual images in a gallery.
.wp-block-image figcaption {
font-size: .8em;
margin-top: .25em;
margin-bottom: 1.25em;
text-align: center;
}
However, I cannot seem to center align the caption that appears under an entire gallery.
Note that this is not on my live site in the URL below — I’m working on a staging site. See screenshot https://www.dropbox.com/s/km4fypy7hda1c3z/captions-staging.png?dl=0