These APIs are notoriously hard to get right and coherent, and releasing at such an early stage will make it hard to change anything when building up on this.
That said I do not know of any cross platform library in C, so this does seem to fill a niche.
Plus it's MIT licensed. You can fork this easily and maintain its behavior the way it suits whatever you're building even if the upstream changes in breaking ways.
Yes. GTK on Windows is borderline useless unless you pour significant effort into optimizing your program for it; and under OSX no amount of effort will make it feel anywhere close to native.
I wonder how Deluge looks/feels on Windows or OS X. I've heard only good things about its native looks, but haven't ever used it on anything but Linux.
Gtk (at least from 3 onwards) doesn't even aim to be cross-desktop on Linux, let alone cross-platform. Meaning, your gtk apps will always look and behave according to the Gnome conventions on all platforms, which could be what you want, depending on what cool-aid you're drinking ;) .
That said I do not know of any cross platform library in C, so this does seem to fill a niche.