1000 megawatts
and before you ask, 1 megawatt is 1000 kilowatts and so on.
So 1 gigawatt is 1,000,000,000 watts (1 billion). Not a measurement that is used often outside of hollywood. Even the largest power plants are in the 1500 megawatt range and no one calls them 1.5 gigawatts.
While we are on the subject of units, yes the whole world did the conversion some years ago but other than Soviet bloc nations, very few were able to do so completely. The UK and much of Europe still measure process piping and instrument tubing in inch sizes, Pressure ratings in ANSI or API psi rating classes, oil in barrels and much of the world still uses HP for engine power.
Here is a trivia question: How many gallons in a barrel? Trick question.......