Specifically, a unit of Gwei is defined as one-billionth (one Nano) of an Ether. So 1 Gwei equals 0.000000001 or 10-9 ETH. Conversely, 1 ETH equals one billion (109) Gwei.
To better understand the concept of Gwei, we may use an analogy with the US dollar, where Gwei would be equivalent to coins (cents) and ETH to dollar bills. For small purchases, it’s easier to refer to prices in cents rather than in fractions of a dollar. As such, Gwei may come handy when measuring small values, and that’s the reason it is widely used when calculating the gas price.
Although less common, Gwei may sometimes be referred to as a Shannon, NanoEther or Nano. The term Shannon refers to the mathematician and cryptographer, Claude Shannon, who is known as the father of Information Theory.