In this particular package, you can click on the button with the ZIP name located to the left of the git URL to download the zip file. Then unzip it to the working directory. Or you can use TortoiseGit to clone the code into a local directory.
Then you can use install.packages with type='source' .
install.packages("duncantl-RGoogleDocs-0e3879c/", type='source', repos=NULL)
But, in general, it would be nice to learn how to create packages.
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