Hi! I needed server-side KaTeX in Ruby a while ago and maintain this gem: https://github.com/glebm/katex-ruby (predates sskatex by about 10 months).

It has some differences: the assets are bundled with the gem (allowing for a simpler API), and Rails is supported out of the box.

I also use my gem with kramdown (for Thredded): https://github.com/thredded/thredded-markdown_katex/blob/master/lib/thredded/markdown_katex/kramdown/katex_converter.rb