"We already have HTML". Yes, but with Gemini, you know the entire "Geminispace" is going to be like that. It's nice for the state of mind! It also ensures sites on the network won't track you, as the wealth of HTTP headers can kindly assist with.
"We already have Gopher". Yes, but Gemini comes with some very useful, almost essential, additions for broader adoption like mandated TLS encryption and things that should have been solved decades ago like defining character encodings, use of MIME types to define binaries, and well supported links to non-Gopher protocols.
