How to install RGtk2 on OS X 10.10.5?
Seems like a problem with RGtk2 which does not support Quartz GTK on OS X (and provided by Homebrew). I followed the advice posted here:

https://github.com/Homebrew/homebrew/issues/43290

Which suggests installing the binary version:

install.packages("RGtk2", dependencies = T, type = 'mac.binary.mavericks')