Certain combinations of R and Mac OS X have trouble with this ( details on the problem and instructions on how to work around it below). With some versions of R > 3.1 and Mac OS X, GTK+ now works out of the box with the following installation instructions.
in installations from GitHub using devtools, try: install_github(., reload=F, quick=T). If the other package depends on RGtk2 and installation from source fails during package test loading with an error that RGtk2 could not be found, try installing without automatic reload, e.g. Note: for some reason on Windows 8, the RGtk2 package is not always recognized during compilation of other source packages, even if it can be loaded and works perfectly fine. Restart R/Rstudio and confirm GTK is now up and running by reloading the package: library(RGtk2).Afterwards it will likely still complain (restart required). Choose "Install GTK+" when prompted, it might take a few minutes to install.
This will notice the missing GTK and prompt you to install it.