If it has ants, it's a near sure bet it has other pests - aphids, scale, or mealies.

Most ants are easy to eliminate. Just buy some of the syrupy ant baits & place nearby (away from kids/pets). The ants will carry the bait back to hill/nest & eliminate the colony. I would look very carefully at the plant to see if there are other pests. Once you determine what they are, you can formulate a plan of remedial action or come here and/or to the pest forum for advice.

Al

I agree with Tapla about the ant baits. Just make sure that you purchase a product that says BAIT on the packaging and that you read the label carefully and follow the directions exactly. Many people who use one of these baits for the first time are horrified at the initial ant activity the bait causes. Their reaction is to start killing ants! In actuality, ants will surge the bait tray, taking the substance to the nest, where in a short period of time, all members will be in ant heaven. You should not disturb the bait tray until all ant activity has been still for at least 24 hours.

The ants are most likely attracted by honeydew secreted by aphids which are feeding on your gardenia. An insecticidal soap spray will deal with the aphids.