So, because I am a nub in this stuff... When it says C library, does that mean anything that works with C (say a gui library for example GTK) will work perfectly fine with this? I would just change the syntax as required and call it good?
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).