What is a gigadollar?

Answer 1

In the USA, a giga-something is 1 billion of something, for example, a gigabyte is 1 billion bytes. Similarly, a gigadollar is 1 billion dollars.

Most elsewhere, a US billion is called a miljard, and a billion is a US trillion. To avoid confusion, use the metrix prefixes like kilo-, mega-, giga-, tera-, etc. or scientific notation, e.g. 1 gigadollar is 1e9 USD.