How many gigabytes are in a gigawatt?

Gigabyte and gigawatt are unrelated.
Gigabyte is used to measure the capacity of memory used in computers. It is either 1 billion bytes (10 to power 9) or approx 1.07 billion bytes (1024^3) depending on the context. In case of RAM the binary definition is used and the decimal definition is used in other cases.

https://en.wikipedia.org/wiki/Gigabyte

The term gigawatt is a unit of power equal to 10^9 watts.

https://en.wikipedia.org/wiki/Watt

I would have to agree with sameer638 (above) that the two units of measurement are not compatible.

- giga, meaning 1,000,000,000 (one billion) 10^9 (Gigawatt, 1,000,000,000 watts)

The MB, or megabyte, mega meaning one million. Seems logical that one mega (million) byte would be 1,000,000 (one million) bytes. It's not however. One megabyte is 1024 x 1024 bytes. 1024 kilobytes is called one Megabyte. So one kilobyte is actually 1024 bytes, and 1024 of those is (1024 x 1024) 1048576 bytes. In short, one Megabyte is really 1,048,576 bytes.


http://www.smartftp.com/support/kb/bits-bytes-mega-giga-tera-f53.html

Simple answer: none. It's like asking how many carrots there in a diamond (note spelling), or how many fish there are in a candle.