When using gedit with syntax highlighting mode set to “Octave”, it erroneously does not highlight the Octave function “uniquetol”, part of the Octave language for 18 months. I request you to add “uniquetol” to the list of Octave keywords. If this is user-configurable, please point to how to edit keywords ourselves.
Yes, the syntax highlighting definition files are located in the GtkSourceView library.
But in the meantime, you can also copy manually the *.lang file inside a home directory that gedit will take first instead of the GtkSourceView one. As usual, see the docs, so the directory is: ~/.local/share/gtksourceview-4/language-specs/. (gedit uses GSV 4, and its API version is just “4”).