Gold is a noble metal. It does not react with many kinds of chemicals readily. However, it does react with chlorine and some other acids slowly. It does dissolve in Nitric acid and Hydrochloric acid mixture called aqua regia. It gets slowly oxidized over years.
If copper or silver is mixed in Gold ornaments, then they do react with acidic chemicals and gases slowly.
Gold also collects some dirt and impurities over its surfaces. In the cleaning processes, possibly ammonia, aqua regia or some solvent is used to dissolve all impurities collected on the surface. So a little part of gold also gets dissolved. If Gold consists of copper also, then some copper also gets dissolved.
So during the purification or shining or polishing act, some metal of the ornaments does gets removed. So its weight decreases.