I agree it would be nicer to have a subset of HTTP and HTML/CSS (not sure if JS could ever be made safe) but as cookies show, the simple fact that headers exist

I agree it would be nicer to have a subset of HTTP and HTML/CSS (not sure if JS could ever be made safe) but as cookies show, the simple fact that headers exist in HTTP and can be extended, makes it a privacy-problematic protocol from the start. The transport protocol used by Gemini is protected via TLS, just like HTTPS. It has the great advantage that the only way for a user to identify itself is by using mutual TLS, which is basically the only 100% secure way to do it (the fact that it's impossible to use mutual TLS on the web tells a lot about the incentives going on there). Gemini absolutely does not re-invent the wheel here.