Q.tropical indoor shrimp and gardenia
My gardenia and shrimp plants have very sticky leaves. What is causing this? I have brought them indoors for the winter.

You likely have a pest infestation and the sticky substance is the residue of the pests; called Honeydew.
Treat the plants with Neem Oil.
Here are some links with more information.
https://www.gardeningknowhow.com/garden-how-to/info/what-is-honeydew.htm
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm
https://www.gardeningknowhow.com/ornamental/flowers/gardenia/gardenia-pests.htm
https://www.gardeningknowhow.com/ornamental/shrubs/shrimp-plant/growing-shrimp-plants.htm