GitLab now supports support for [math in markdown][gitlab-math] files both in wikis using the syntax:

 $`E = mc^2`$

for inline math, and fenced code blocks with the language name math for block math:

 ~~~math
 E = mc^2
 ~~~

Unfortunately, this is an idiosyncratic syntax that is not yet supported by any existing Markdown editor that I am aware of. Most Markdown editors do not support math at all, but those that do usually just support standard LaTeX embedded math syntaxes (e.g. $...$, \(...\), \[...\]).

However, editing tools that are based all or in part on JavaScript may be able to handle math rendering the same way that GitLab does by using the KaTeX JavaScript math rendering library, which supports both server-side and in-browser rendering of math. There is an associated auto-rendering extension script that can search for unrendered math markup in a DOM and replace it with rendered versions. With a little extra bit of JavaScript provided in this project it is possible to support GitLab syntax.

LightPaper is an inexpensive (but not free) Markdown editor for Mac OS X 10.10 or higher. While it does not natively support GitLab syntax, it allows you to specify CSS files and JavaScript scripts to be automatically be loaded for all documents, and this can be used to install code to render GitLab math in place.

Here is what you need to do:

1. Download KaTeX from here and install it somewhere on your machine. (Choose tar.gz or zip file)
2. Download gitlab-katex.js from this project and install somewhere. Or you could simply just clone this repository.
3. In Preview tab of LightPaper preferences make sure that Enable Math Rendering (powered by MathJax) is turned off. We will be using KaTeX instead.
4. In Script Assets tab of LightPaper preferences, add the following entries:
   • katex library, which is katex.min.js in the KaTeX download
   • katex auto-rendering script, which is contrib/auto-render.min.js in the KaTeX download.
   • gitlab-katex.js] script downloaded from this project
   • katex CSS file, which is katex.css in the KaTeX download (this is NOT optional)
5. In Script Assets tab of LightPaper preferences, select On Update Callback Script at bottom of dialog and add renderGitlabKatex(); to body of update function.
6. Restart LightPaper

KaTeX is not a complete implementation of LaTeX math, so you may want to occasionally update KaTeX to get the latest updates.