Jim, what you have is a U S commemorative half dollar that was issued in 1982. It was NEVER issued gold plated ! This was done by some other co.
The current value for this coin in only $7.00. The stamp is worthless as it was not issued with this coin. There were two types of this coin issued. One was a proof that came in a maroon box, and the other was uncirculated that came in a green box. Hope this helps you ! Ken