The gas given out by green plants during photosynthesis is oxygen (O2).

Here's why:

* Photosynthesis is the process where plants use sunlight, water, and carbon dioxide to create their own food (glucose) and oxygen as a byproduct.

* The chemical equation for photosynthesis is:

6CO2 + 6H2O + sunlight → C6H12O6 + 6O2

* This equation shows that carbon dioxide (CO2) is used up and oxygen (O2) is released.