Question
There is a sticky substance on my Gardenia.  The buds are also affected and are turning brown and dropping.  

Answer
I would think your gardenia has either scale insects or aphids. Look for small bumps on the twigs or leaves--if these exist then the problem is scales, Or look for small clear bodied insects that crawl--these are aphids. Both suck the juice out of the plant and deposit a sticky substance calle "honeydew". Spray the plant with an insecticide called Malathion. There are other insecticides that will kill aphids also just check labels for "aphid and scale control". Make sure you wets both side of the leaf when spraying. There are insecticidal soaps that will also work on aphids. Check with your local garden store for an aphids controlproduct.