Quincy and Gwendolin can use their abilities to damage their own Golden Bloons and get extra Gold.
Mortars have no control over attacking a bloon at the end of a turn. If only Golden Bloons are on screen, you can force the opponent to give you more Gold.
You can punish an opponent Quincy's Perceptive Shot by sending a Golden Bloon. The card would essentially cost 1 Gold when played this way, and you get the same Bloontonium.