Gardenia plants do love acidic soil when growing so adding coffee grounds to the soil for Gardenia Plants can help them grow healthy and strong and keep them thriving.
You can mix the coffee grounds in with the soil directly or add some coffee grounds to the compost and then use the compost with the coffee grounds to plant the Gardenia Plants in.
Just avoid putting too many coffee grounds around the gardenia plants to avoid causing the soil from becoming too acidic which could kill the gardenia plants.
But in moderation Coffee Grounds are wonderful for growing Gardenias.