To keep score in Gold Fish, players count the number of sets they have collected at the end of the game. Each set is worth one point. If a player has more than one set of the same rank, they receive additional points for each set. For example, if a player has three sets of sevens, they would receive three points. The player with the most points at the end of the game is the winner. It is important to keep track of the score throughout the game to determine the winner accurately.