Because this Darth Vader doesn't regurgitate water...

... he is not a gargoyle - even though he gazes down from the Washington National Cathedral as a decorative element.

To be a true "gargoyle," you have to, well, "gargle" -

The term originates from the Frenh gargouille, originally "throat" or "gullet"; cf. Latin gurgulio, gula, and similar words derived from the root gar, "to swallow", which represented the gurgling sound of water (e.g., Spanish garganta, "throat"; Spanish gárgola, "gargoyle").

A chimera or a grotesque figure, is a sculpture that does not work as a waterspout and serves only an ornamental or artistic function. These are also usually called gargoyles in layman's terminology, although the field of architecture usually preserves the distinction between gargoyles (functional waterspouts) and non-waterspout grotesques.

There's more information on this grotesque at the Washington National Cathedral's website.

How did the Star Wars villain get there? Turns out in the 1980s, the Cathedral sponsored a sculpture design competition for children. Four winning designs were chosen: a raccoon, a girl with pigtails and braces, a man with large teeth and an umbrella, and lastly, Darth Vader.

The Wikipedia entry says the cathedral also sports a robot and... a crooked politician!