Question: how to install rgtk2 on mac?
Answer: From the R command line (e.g. in R-Studio), install the RGtk2 package by running: install.packages("RGtk2", depen=T) ... 
Then load the package by running: library(RGtk2) ... 
Restart R/Rstudio and confirm GTK is now up and running by reloading the package: library(RGtk2)