Definition of gigawatt:
(n) :
One thousand million (10⁹) watts, an amount of power large enough to power such things as a midsize town or several small ones. (Consuming 1 gigawatt during a duration of 1 hour consumes 1 gigawatt-hour of energy.)
(n) :
(metonymic) The amount of data center capacity or amount of compute that this amount of power can provide under the current technological state of the art (PUE, clock speed, etc).