Scale Infestation

This appears to be two types of scale, (brown scale and cottony cushion scale), but both are sap sucking insects that attach themselves to the leaves and stems. Wiping them off with a damp cloth can be effective if caught early, but yours is more extensive. Organic controls include spraying with horticultural oil being sure to coat all leaf surface, but take outside before spraying. Make sure you spray in the early evening to prevent sun scald. Or cut off the worst of the infected leaves if it will not defoliate your plant. If you wish to undertake chemical control, look for a systemic formulated for scale and for your container gardenia plant. Use according to directions for your plant.
Here is a link that you might find helpful: http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7408.html