If you were going to develop a mono app on osx you'd be better off using monomac than gtk#. Mono develop is written in gtk# and cut and paste works just fine on osx.

My use case is the most basic one possible:

-Build a GUI app in C# -Run in on both my Windows dev machine (Windows 7), and my main desktop (OS X)

That's it. I don't care even what I have to use to build the app (Windows Forms, Stetic, etc).

Any advice?

If your app is something others will use, for the best experience you write your app in such a way that the UI is as swappable as possible. Then write a Mac UI with monomac and a Windows UI with WPF. If it's just you who needs it, then Gtk# might be ok for standard things. But don't expect good os integration without work.

Have you considered using node-webkit, AngularJs, and Bootstrap for your cross platform GUI needs?

The problem with this is you only get 1 thread for the UI and other stuff to run on.