How to change the default highlighting of gedit for a specific extension
Solution 1:
You could change /usr/share/gtksourceview-3.0/language-specs files but instead copy this file to your home or you will lose changes on upgrade.$ cp /usr/share/gtksourceview-3.0/language-specs/octave.lang ~/.local/share/gtksourceview-3.0/language-specs/
change<property name="globs">*.m</property>
to<property name="globs"></property>
I also did some experimenting and found out objc.lang file also has the same line so you maybe need to copy and change that too.