One of the first ever recorded uses of gold in the human mouth was back in ancient Italy (modern-day Tuscany). A people, known as the Etruscans, used gold in the mouth dating back to at least 200 B.C.E. They used gold wire to hold their teeth together, like a modern day retainer or bridge, that was removed while eating.
Another ancient example of gold in teeth was found with the discovery of what is known as the Bolinao Skulls. This was an excavation project unearthed in an island off the coast of the Philippines. Archaeologists excavated 67 skulls, all of which had their teeth decorated in gold. The skulls were dated at 14th or 15th century. Gold teeth were found to be decorative in nature with unique designs for each tooth.
As far as gold teeth in modern dentistry, the first known practice was by early dentists in the 19th century. They used gold as fillings after drilling a cavity. This is because gold is so malleable and doesn't decay over time when exposed to liquid such as saliva.
If you are thinking of replacing those gold fillings with something with a little less "bling", you can exchange that gold for porcelain or silver amalgam, and fetch anywhere from $40 to a few hundred dollars depending on the size. It might help offset that dental bill!