Here's the breakdown of the reactant gas and product gas of photosynthesis:

* Reactant Gas: Carbon dioxide (CO2) is the reactant gas in photosynthesis. Plants take in carbon dioxide from the atmosphere through tiny pores called stomata on their leaves.

* Product Gas: Oxygen (O2) is a product of photosynthesis. During the light-dependent reactions, water molecules are split, releasing oxygen as a byproduct. This oxygen is then released into the atmosphere.