Hi @cpeters,
No, it is not currently possible to create a galley only through the API. Our API does not have full coverage of all the functionality that is present in OJS, but we are interested in hearing more about what aspects of the API users would like to see expanded.
There is already a post about this on the feature request side of the forum, so you could add any additional thought to that and/or upvote it.
Regards,
Erik
PKP Team