It is much more lightweight and simpler: While you can, but typically won't write HTML directly with a text editor, you can and typicall would write gemtext markup manually like markdown.
You can run it on a small rasberry pi, using a server like agate, which automatically generates certificates, and if you fancy, you can write your own client browser in a weekend (that's why there are a lot of clients).
It is very deliberately designed to be minimalistic, to depart from the enshittified modern web flooded with ads, tracking, and slop.
And because of this, pages load instantly.
Myself, I have witnessed the birth of the World Wide Web, designed by Tim Berners-Lee at CERN, to share information, new ideas and knowledge, and run the first Mosaic browser in 1994, and gemini is very close to the hearth of this original idea.