Without an argument returns the name of the normal font being used. Like what is used for the Normal highlight group |hl-Normal|. With an argument a check is done whether {name} is a valid font name. If not then an empty string is returned. Otherwise the actual font name is returned, or {name} if the GUI does not support obtaining the real name. Only works when the GUI is running, thus not in your vimrc or gvimrc file. Use the |GUIEnter| autocommand to use this function just after the GUI has started. Note that the GTK GUI accepts any font name, thus checking for a valid name does not work.