Gold Plating can simple be seen as a processing of clothing some amount of gold over another metal,especially on the surface either by chemical or electrochemical means. Therefore, Gold plated rings are simply being clothed with little layers of gold on the surface of another metal such as copper, etc.
For a gold plated ring to tarnish, it all depends on the amount/thickness or layers of gold being used during its plating on another metal. Gold plated Rings are also beautiful and crystal gem stones can be placed on them during its manufacturing. Gold plated rings may turn brownish in color during its tarnishing period.