So, I looked into how I could add text highlighting for a currently unsupported language in gedit. The files that define the syntax highlighting for gedit are located (for me at least) in:
The language files have the extension
*.lang. After trying to create a TypeScript language file myself with minimal success, I finally managed to get the right Google search of “gedit lang file for typescript” and found that someone had already created one. It’s on github here.
I then just downloaded this file, placed it in the directory above (using
sudo cp in the terminal to do so), restarted gedit, and it worked!