A small fraction of the $2.50 Amazonian patterns in copper (J-1231's) were gold-plated at some point in their life. It is difficult to determine if this was done at the Mint or privately. The plating was done to give collectors a chance to own a coin that, at the very least, looked like the unique gold Amazonian $2.50. We know of two Gilt J-1231's -- see the PCGS CoinFacts Condition Census listing above -- both of which are now in the celebrated Simpson Collection.
J-1231 was part of a set of pattern coins known as "Amazonians", so-named because of the unusual, warrior-queen motif on the obverses of the 25C, 50C, and $1 (silver) coins. The $1 (gold), $2.50, $3, $5, $10, and $20 are also called "Amazonians" despite the fact that their obverses show only a head of Liberty. However, there is a direct connection between them all because of the identical reverse design.