Slides
The I() construct
?formula gives you insights into the way to structure generic formula objects in R. The I() construct is a consistent and clear way in R’s foundations to specify functional operations on variables.
R and GMM
There are a number of packages for estimating GMM models in R. pgmm does many of the most basic models, though Stata is the industry leader in this. That said, there is a relatively new and highly customizable package for this: pdynmc which is described here in a pdf vignette.