Rstudio cloud only has direct access to its own filesystem, not your local machines c:, and not the internet.
However, for downloading an available resource from the internet you can tell R that what you want comes from a url by wrapping with url()
deck<-read.csv(ur("https://gist.github.com/garrettgman/deck.csv"))
this would work if the csv was available at the url; it isn't available for me, its probably a private i.e. non-public file.
Here is an example of some csv I found that is publicly available
deck<-read.csv(url("https://raw.githubusercontent.com/rengalv/Kaggle-Titanic-in-R/master/test.csv"))
Note that I've found that typically I need to view a csv in 'raw' mode on github in order to get an appropriate link to pass to R etc.