This task needs more info, i.e. why does this need to be exposed?

Sorry, this was a stub.

In T110910, we're writing a gallery implementation natively in Parsoid.

In order to render them correctly, we need the defaults from the wiki.

See the FIXME on L35 here, https://gerrit.wikimedia.org/r/#/c/264026/34/lib/ext/Gallery/index.js

Change 322973 had a related patch set uploaded (by Arlolra):
Expose $wgGalleryOptions in the MW API

https://gerrit.wikimedia.org/r/322973

Change 322973 merged by jenkins-bot:
API: Expose $wgGalleryOptions in the meta=siteinfo

https://gerrit.wikimedia.org/r/322973