It depends on numerous factors such as its country of origin, denomination, condition, mintmark (if any), errors, etc so you need to be much more specific. If you mean just its actual gold content value that changes daily so you will need to look up the current value of gold and then figure it out from there.
You bought a gold coin for one dollar? I am assuming it is not pure gold because pure gold costs hundreds of dollars(even thousands), not one dollar. The coin is probably another type of metal painted gold. So it is worth exactly what you paid.
Actually its very difficult or not possible to answer it right away. First you need to answer these questions What is its denomination? What is the date of the coin? Does the coin have wear or tear? What country has issued? Does it have a mint mark, if so what is it?
Without the answers to these questions its not possible to answer your question. Find out more about gold bullion bars https://store.sprottmoney.com/ and a lot more.