One way is
gdoubleprime = Import["/Users/gag/Downloads/data.csv"]. However this will only work for the user gag. A more general way is to use the home directory and downloads with
gdoubleprime = Import["~/Downloads/data.csv"].
You should also consider one of a couple of possible general policies for data files. The first is to put them into the same directory as the notebook, so that they move together and can be placed anywhere on your drive (or on any other operating system for that matter). You could then set the directory to the notebook directory and import from there:
A variation on this that I use (perhaps overkill) looks like this:
dataDir = "../Curated Data";
nameS = “/replace_with_file_name_to_import";
extS = ".csv";
fileNameS = dataDir <> nameS <> extS
This allows you to have multiple MMA project directories all of which look into a single directory at the same level in the directory hierarchy which contains vetted data. The extension string is optional, but I sometimes have xls, csv, etc. files all with the same "left" name and this makes it a little less likely that I will confuse them since the extension string is explicit.