Final answer:
The variable Gary is testing in his experiment is the amount of water provided to plants, which is aimed at determining its effect on plant growth.
Step-by-step explanation:
The variable being tested in Gary's experiment is the amount of water. This experiment aims to investigate how varying levels of water affect plant growth. Gary has established controls for other variables such as the size of the pot, the amount of plant food, and the type of soil to ensure they do not affect the outcome of his experiment. Consequently, the only factor varying is the amount of water given to the plants, which is the independent variable. Other potential uncontrolled factors that could influence plant growth include individual plant variation, soil temperature, water loss due to evaporation, and the effect of insects.