"Goldkapsel" is a gold cap. It is used as an unofficial, internal quality level mainyl for Auslese. Auslese Goldkapsel is usually richer, sweeter, and more expensive than the ordinary version. Often it is more botrysized too. The term is never mentioned on the label, but teh Goldkaplse bottlings are identified by the gold cap only.
As a refinement on the above, for some estates there are more than one gold capsule wine in a category, such as Auslese or Eiswein or even Spätlese. When this occurs the only sure means of identification is the AP number appearing on each bottle of quality wine (qualitätswein) since 1971. Sometimes there are long gold capsule wines (LGK for short), too, which may be difficult to distinguish except if one happens to have a sample of each type of bottle to do a direct cojmparison. Here too the AP number can help if you have a list of what was put out under what number.
All this happens frequently enough to make it desirable to have a list of every AP number used by every bottler in every vintage year, but as far as I know this is not available, nor is it likely to be anytime soon. Sometimes the producer website can help, and another help is offen David Schildknecht of the Wine Advocate, who tries to list all AP numbers where there is likely to be a need for them. This latter note jht.