The Gold Medallion is a key item in the first game of the modern Ninja Gaiden series, found and used in "The Path To Zarkhan" chapter. Once both Gold and Silver Medallions are acquired, they are to be placed on their respective locations on an altar found near the previous save statue. Once placed they will cause an explosion opening a path to the next area for Ryu to proceed.
Both Gold and Silver Medallions are missing in Ninja Gaiden Sigma, and are replaced with the Key Of Triton (found in sunken ship's hidden compartment) which allows Ryu access to the same area.
The Gold Medallion can be found near the beginning of The Path to Zarkhan chapter. Right at the beginning of the chapter if Ryu swims under water, turns to the right and swims under a gap in a gate, he will enter a small area with a treasure chest on the floor containing the Gold Medallion.
In Ninja Gaiden Sigma the Gold Medallion, plus the treasure chest it is obtained from, do not exist.