The Golden Bloon itself is a moderate early-game threat, equal to a +20 HP Blue Bloon or a 2-delay Swarm Green Bloon, and the extra Gold gained per hit can discourage the opponent from feeding Gold to the player.
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.
Against Quincy, his Perceptive Shot passive will activate the on damaged effect.
The card would essentially cost 1 Gold when played this way, plus the same amount of Bloontonium.
It might be helpful to tank Golden Bloons, or delay attacking them, but only if you know the opponent won't be able to afford anything that would save themselves.
Attacks that deal at least the Gold Bloon's health will ensure that the sender will only get 1 Gold, no more.
For wholescreen options, Firestorm one-shots Golden Bloons if not playing on Dark Castle.
Sniper Monkey one-shots Golden Bloons if they're not health-buffed by cards or Zee Jay's passive.