Q.what disease could my gardenia tree have?
I bought a gardenia tree this spring and planted it in a large container outside. I recently brought it into the house for the winter. I have noticed a web like material on the leaves and tree trunk. I sprayed it with an organic peppermint oil and cleaned all leaves and trunk. I removed some leaves with some whitish stuff on top and some brownish on the bottom. The web like material is coming back on the new growth. Can you help?

Certified GKH Gardening Expert
These are spider mites. Their presence indicates that the plant is stressed. This article will help deal with them:
https://www.gardeningknowhow.com/plant-problems/pests/spider-mite-detection-and-spider-mite-natural-control.htm
That being said, as I mentioned, they are an indication of a larger problem. Most commonly with gardenias brought indoors the problem is too little humidity. You may want to look into setting up a pebble tray for the plant. More information on this:
https://www.gardeningknowhow.com/houseplants/hpgen/raise-humidity-for-houseplants.htm