Older leaves of gardenia do yellow naturally. They do not like to be water logged and they do like to be kept evenly moist. It looks like they are in partial shade which is o.k. They are an understory tree and don't mind shade from afternoon sun.
Epsom salts are for magnesium you only need a tablespoon per gallon and only once a year and only if it is needed. Micronutrients like iron, magnesium, boron, selenium are usually included in fertilizers for acid loving plants like citrus, gardenia, and roses. Potting soil for gardenias should have had some starter fertilizer and sulfur to keep it acidic, so you should be o.k. with fertilizer at least for three months.
It could be a little transplant shock so the roots are not taking up enough nutrients and so the plant is dropping leaves to lighten the load. When the roots recover, it should stop. Be careful not to over water. The plant will not be able to take up much water if it has some transplant shock so you want to keep it moist but not soggy or the roots will rot. Gardenias like most tropical plants are not deciduous but they do drop a few leaves at a time normally.
You can have the soil tested later especially for pH. Gardenias do not like alkaline soil. If baby's breath grows well for you, then your soil is alkaline, if you can get blueberries to grow then it is acidic.
Happy gardening in Hawaii. Gardens are where people grow.