Step 1: Recall the photosynthesis reaction. The overall equation is: \[ 6CO_2 + 6H_2O \xrightarrow{light\;energy,\;chlorophyll} C_6H_{12}O_6 + 6O_2 \]
Step 2: Identify the released gas. In this process, water molecules undergo photolysis (splitting of water) and release oxygen gas.
Final Answer: \[ \boxed{Oxygen \,(O_2)} \]