German wine is confusing - what can we say? Let's try to simplify a touch...
A Gold Capsule for a wine is sort of like getting a "gold star" back in grade school.
German winemakers give these only to certain bottlings they think are very special.
See, in Germany, a number of different wines are often made from the same vineyard. Technically, the only way to legally differentiate wines from the same vineyard is by the AP number - it's like the social security number every wine gets. It's why, for example, you'll see a Graacher Domprobst Spätlese #12 and a Graacher Domprobst Spätlese #5 from a producer like Willi Schaefer. The #12 and #5 are the AP numbers.
But if a producer feels a certain bottling is very special, he/she can also give the wine a "Gold Capsule" - literally. In German, the word is "Kapsul," so you'll often see this abbreviated to "GK."
GKs usually have higher levels of ripeness and are both more expensive and much rarer than their normal counterparts.