The best Fertilizer I find for my gardenia plants is Miracle Grow Performance Organics All Purpose Plant Nutrition fertilizer which is formulated for plants that love acid soil.
But you can still add some coffee grounds to the gardenia plants to help them grow.
Gardenia Plants need a fertilizer that is specially formulated for acid loving plants which Gardenias are.
Gardenia Plants just like azaleas, are acid-loving plants so you should buy some fertilizer that is specially made for acid loving plants.
Or you can mix some other stuff to the other fertilizer to add acid to the soil such as through using acidic materials such as coffee grounds, tea or Epsom salt.