gtkmm 3.0 for Mac could be downloaded from the developer's website when we last checked. We cannot confirm if there is a free download of this app available. Gtkmm for Mac lies within Developer Tools, more precisely IDE. We recommend checking the downloaded files with any free antivirus. This free Mac app was originally designed by gtkmm Team.
From the developer:
gtkmm is the official C++ interface for the popular GUI library GTK+. Highlights include typesafe callbacks, and a comprehensive set of widgets that are easily extensible via inheritance. You can create user interfaces either in code or with the Glade User Interface designer, using Gtk::Builder. There's extensive documentation, including API reference and a tutorial.
You may want to check out more Mac applications, such as JBuddy Bot Builder, Objective Basic or Pleinpot, which might be related to gtkmm.