Currently, Midnight Pub doesn't support posting via gemini protocol, because posting needs authentication, which is currently made via HTTP (either from a browser or from a shellscript).
In order to accept authentication via gemini and, by extension, posting via gemini, Midnight Pub would need to have a link between TLS client identities and registered accounts, so a client identity would be recognized as being a specific user.
But there's another, current, limitation of gemini: the payload length (e.g. the post's content or a reply). The payload is sent as a querystring within the URI, and the entire URI should be less than 1024 characters. Characters are often escaped (e.g. space becomes %20) so characters often consume more than 1 character at the URI. This means that posts and replies through gemini protocol would need to be shorter than HTTP currently allows.