You can write regular C GTK+ code side by side with it, but you would need a wrapper in order to use most of the constructs with GTK+, or anything else not writ

You can write regular C GTK+ code side by side with it, but you would need a wrapper in order to use most of the constructs with GTK+, or anything else not written with Cello's type system in mind (e.g. var w = $(GTKWidget) would not work for multiple reasons).