The Gold-Merkliste is only available for Gold-Trainer. It is a cross Setup shortlist to which up to 250 players can be added that you might be interested in signing for more than one club under your control.
Whilst adding a player to your club's shortlist, you also have the option of adding them to your Gold-Merkliste and this is done by selecting Add to Gold Shortlist? The player is then added to a predefined group based on the properties of the player, for example 90+ Rated Players.
In order to receive Club Messages about the players on your Gold-Merkliste make sure you have added them to a club's individual shortlist as well.
The benefit of a gold shortlist is it saves time that would otherwise be spent adding the same player to the shortlist for multiple teams. It also simplifies the process of creating a new shortlist for any new club that you take control of as there would already be a predefined shortlist available.