The original authors of GTK+ were Peter Mattis, Spencer Kimball, Josh MacDonald.
Since then, much has been added by others. Please see the AUTHORS file in the distribution for the GTK+ Team. The people currently contributing the most code to GTK+ are (in no particular order): Owen Taylor, Matthias Clasen, Federico Mena Quintero, Soeren Sandmann, Padraig O'Briain, Manish Singh, Kristian Rietveld, Tor Lillqvist
http://www.gtk.org
" GTK+ is free software and part of the GNU Project. However, the licensing terms for GTK+, the GNU LGPL, allow it to be used by all developers, including those developing proprietary software, without any license fees or royalties."