I created an R package to read grads data. As far as I know, there is no dedicated package to read grads data. The package is still quite new, any remarks on the documentation or code are more than welcome. If you are interested in this R package, please leave a comment below. In the future I plan to release the package to CRAN.
To install the package you need to get the code from my bitbucket account page (I use mercurial) and compile the package from source. I assume that Mercurial is installed on your machine.
hg clone http://bitbucket.org/paulhiemstra/readgrads
R CMD INSTALL readgrads