I\'ve installed R3.0 on my Mountain Lion Mac and after some trial and error the version of GTK suggested by http://r.research.att.com/ for R3.0: GTK+ 2.24.17 framework.
I too had this problem. Here is how I worked around it using macports gtk2 -- not the http://r.research.att.com/ version. This was with R version 3.0.1 (2013-05-16) -- "Good Sport", also compiled from source.
sudo port install gtk2 ## (X11 -- not aqua)
export PATH=/opt/local/bin:/opt/local/sbin:$PATH
R CMD INSTALL RGtk2
R CMD INSTALL cairoDevice