Elodea, the water plant? Well if you are doing the underwater elodea plant lab, I can help you because I did the exact same thing a few weeks ago. So basically, it is showing the effect of photosynthesis. I will get straight to the point, but if you have any other questions, I can help you with them as well, since I personally loved this lab. Since elodea is a plant, the gas being produced by it is oxygen.
Hope that helped, and leave any other questions in the comments! Can I have brainliest, I'd love it :)