When a gold entity is detected, the JSON returned by the NLP API is enriched with additional information about the entity. Although you can't change the enrichments for gold entities, you can now create enrichments for custom entities. This means that you can create keys that appear in the JSON when a custom entity is detected. The keys are then populated with the correct values, depending on the configuration.
For example, you have a list of train stations that you want to detect. You create a restricted custom entity with all the names of your train stations. Now you add the key station-code and configure it so that if Grand Central Station is detected, the JSON also returns the station code GCT only for this station.