looks like rendering adblockers extensions obsolete with manifest-v3 was not enough so now they try to implement DRM into the browser giving the ability to any website to refuse traffic to you if you don't run a complaint browser ( cough...firefox )
here is an article in hacker news since i'm sure they can explain this to you better than i.
and also some github docs
It is static pages with hyperlinks, only in a different protocol. It's supposed to be like upgraded Gopher with some good things from modernity and HTTP.
Static pages with hyperlinks have evolved into a certain horror we all know. One of the stated goals is that Gemini is not extensible by design. It's not intended to easily grow additional features, even server-side theming of pages.
Why new protocol and clients - because of control. It's a small protocol, clients are simple, they don't need all the sandboxing and interpreting and DOM that web browsers have.