Welcome to the forum.
If you are looking to buy gold coins, and you have a $100 to spend, your options will be limited, but you still have some choices. There are half gram and one gram coins available, minted by Australia, China, Mongolia, and other countries. You may also be able to find 1 gram gold bars by Engelhard, Credit Suisse, etc.
While you are there, take a look at some ancient coins, check out some Early American Copper, or Conder tokens, and look through a few reference books. It can be a real learning experience, if you take the time to browse look around.
Have fun!