Creating your own syntax highlighting in GEdit?
GEdit uses GtkSourceView for its syntax highlighting. You should be able to find the c.lang file it uses to highlight C code by typing a command like this:
$ locate gtksourceview | grep /c.langOnce you find the lang file, open it up in a text editor (it's an XML file) and near the bottom you'll see a list of keywords which you should be able to add printf to.
Thanks to Jeremy's post I found this page: http://projects.gnome.org/gtksourceview/documentation.html
Here you'll find a link to both a tutorial and the official reference for the language definition files.
update: Another useful link http://wiki.gnome.org/Apps/Gedit/NewLanguage