Ants eating Gardenias
Does it mean the gardenia bushes I have which the leaves have been totally eaten overnight by ants should be thrown away or can I revive them

Try the ant control recommendation in the articles below.
Your Gardenia may have a chance to recover if you can resolve the issue quickly.
https://www.gardeningknowhow.com/houseplants/hpgen/ants-in-houseplants.htm
https://www.gardeningknowhow.com/plant-problems/pests/insects/keeping-ants-off-flowers.htm
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