Gold leaves are manufactured from real gold and beaten into ultra thin gold leaves. It takes a relatively small amount of gold to gild a large surface.
Gold leaves are available in different karats. 24 karat is pure gold, other karats represent 1/24 of the pure gold. 12 karat gold leaves contain 50% pure gold. All versions of gold below 24 karat are alloyed with other metals. For example: 18 karat Lemongold ("Zitrongold") contains 18/24 gold and 6/24 brass, red gold 23 karat contains 23/24 gold and 1/24 copper.
For exterior application of gold leaves, only use leaves that contain 23 karats or higher, because the alloyed metals oxidize and drastically change the color tone.