No inline images is not a restriction of the protocol. It's a restriction of the text/gemini MIME content type, and of the browser implementations. A server can still respond with text/html content over the GEMINI protocol, with embedded <image/> data. The GEMINI protocol specification does not restrict what RFC2045 types can be used.

* https://geminiprotocol.net/docs/protocol-specification.gmi

* https://iana.org/assignments/media-types/media-types.xhtml