* Nano (n) means one billionth (1/1,000,000,000)

* Giga (G) means one billion (1,000,000,000)

Therefore, there are 1,000,000,000,000,000,000 (one quintillion) nanometers in 1 gigameter.

Here's the breakdown:

* 1 gigameter (Gm) = 1,000,000,000 meters (m)

* 1 meter (m) = 1,000,000,000 nanometers (nm)

So: 1 Gm = 1,000,000,000 m * 1,000,000,000 nm/m = 1,000,000,000,000,000,000 nm