This syntax file should help you working/looking at Constraint Grammar files from Apertium with gedit. To use this syntax file you need to add lrx.lang to the directory ~/.local/share/gtksourceview-3.0/language-specs You can do that with the following command:
wget https://raw.githubusercontent.com/nuboro/syntxfile_gedit_CG/master/rlx.lang -O ~/.local/share/gtksourceview-3.0/language-specs/rlx.lang
If you haven't got that directory first create it with:
mkdir -p ~/.local/share/gtksourceview-3.0/language-specs
Now you can start using the syntax file either if you choose "CG" or automatically if the file ends with .rlx