Answer:
1000. A megawatt-hour is the energy you'd get with a megawatt of power for 1 hour. Energy = power * time. It's important to note that it's 'megawatt hour' and not megawatt/hour (ie. it's not megawatt per hour).
A gigawatt has 1000 times the power of a megawatt. Thus, since the time is the same (both 1 hour), a gigawatt-hour = 1000 megawatt hours.