Art Leah points out the difference also involves an intermediate layer between metals, and why, and tells a personal story:
"In gold-filled metal there is an intermediate layer of gold alloy between the gold and the basic metal (which is why it’s called “gold-filled”). This layer is missing in gold-plated metal.
The intermediate layer plays a key role in the durability of the gold look. Gold tends to “diffuse” or mix in with the atoms of other metals. In gold-plate this intermingling happens rather easily, allowing the base metal to rise to the surface and causing the gold look to fade. The intermediate layer in gold-fill, made of a blend of gold and brass, captures and holds atoms in the top gold layer and the bottom brass layer.
This interlocking action makes the gold diffusion process slow and gradual, together with there being more gold to start with! In the long run, the beauty of gold-filled jewelry lasts many years longer, making its higher price more worthwhile.
The lesson of my husband’s cuff links.
When we still lived in Vilnius (Lithuania), I bought my man a set of beautiful cuff links as a gift. They were made from gold-plated silver and inset with rock crystal. He thanked me, put the cuff links back in their box... and never touched them again. Oh, well.
Some years later, already in Israel, I was sorting through things and come across this box. I opened it, and was very surprised to see pure-white cuff links inset with rock crystal. The gold layer had completely disappeared, even though the cuff links had never been worn even once.
So diffusion is unstoppable, no matter how carefully you clean and store your gold-plated jewelry. If you love the color of gold, invest your hard-earned money in gold-filled pieces to avoid disappointment."
http://artleah2010.blogspot.com/2011/12/gold-plated-gold-filled-what-does-it.html#.VMSDNpB0yM8