Metmat, is your plant in a pot or in the landscape? This would be helpful to know for folks to give you the most appropriate advice. What else can you say about it? Is it getting plenty of sun? Has the weather been normal, extra dry, extra wet? How old is your Gardenia?
White flies can fly, so much more difficult to battle. Aphids can be removed from a plant with water and fingers. But I've never found it necessary to battle pests on landscape plants. Pests come and go sometimes, and rarely cause more than slight cosmetic damage to a healthy plant, just part of nature. If one is always killing pests, beneficial predator insects have no source of food, and are often also killed if they've shown up and are also 'on site.' Beneficial predators don't show up *before* pests. Those who would insist every leaf should always be perfect and every pest must be proactively 'handled,' are trying to sell you a package of something. I don't own any packages of stuff like that, and don't find any of it necessary. I also don't think I'm capable of controlling nature, with or without packages of stuff from a store, or that attempting to do such is a good idea.
OTOH, a potted plant can be more at risk. Each package of stuff should say what pests it's known to kill, and what plants won't be harmed by it. Gardenia, being such a popular and ubiquitous plant, would be one of the easiest for which to find a package of stuff that will do the job. Just read the labels, and follow the instructions, if that's what you feel is necessary.
I'm sorry for 'going all philosophical' this morning, and please know it was just that, not personal. I feel strongly that if people would stop trying to micromanage nature, they would be pleasantly surprised by how well the natural order of things 'works' and am sometimes triggered by a perfectly innocent question to share what I believe, my personal opinions, much more info than was solicited.
I hope you enjoy your Gardenia well, and that you find similar peace with the occasional pest, or, if you prefer, a package of stuff that that does the trick!
