Gold-filled jewelry has 100x more gold alloy than gold plated and because that layer is so much thicker, it means gold-filled jewelry lasts longer and stands up to wear and tear better than gold plated. Gold filled is much more economical than solid gold and is best for long wear and sensitive skin.
The layer of gold alloy on gold-plated jewelry is so minuscule it is barely noticeable in the above image. All it would take is one small scratch on a gold-plated piece to expose the jeweler's brass underneath. Due to the thin layer of gold which can easily wear off, gold plated pieces can tarnish once the core metal is exposed