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.
What exactly do you think you are proving? It doesn't matter what some spec theoretically allows, if you want to server something to gemini users it can't have inline images.