I would suggest doing a Google search for ‘macports < your error message >’ as these questions have probably been asked many times before. Do you have X11 available? For example, try sudo port install xorg-server.
If you get an error about D-Bus, see this comment.
GTK is trying to use X, which I do not want .Any ideas how to force it to use quartz? I’m hoping to have a platform independent program when It’s finished. Is that even possible with gtk?
As you’re using MacPorts, this is a MacPorts question. First see this section on port upgrade. The manual suggests you can do
sudo port upgrade --enforce-variants gtk3 +quartz
I haven’t checked that works.
Of course, the Gtk API and the stack of libraries it is built on are portable except - obviously - parts that provide platform-specific features, e.g. the GdkX11 API, but you don’t need to use any platform-specific APIs unless you have a specific requirement relating to a particular platform.