A gigatonne is 1,000,000,000 tonnes, and is often used when discussing human carbon dioxide emissions. This is roughly the mass of all land mammals (other than humans) in the world as per the XKCD comic strip to the right. It's also roughly twice the mass of all of the people in the world.
1 gigatonne is also roughly 200 million elephants; enough elephants to stretch from the Earth to the moon. Which would be bad for the elephant at the bottom and it would take a lot of food to feed all of those elephants. A gigatonne is also equivalent to
The graph below shows how much carbon dioxide different regions emit, note that regions emit many gigatonnes of carbon dioxide every year. This incredible amount of carbon dioxide going into the atmosphere is what is leading to climate change.