Please provide more context! The answer depends on what nuclear reaction you are referring to.

For example:

* Nuclear fission of Uranium: This produces various fission products, but the most common gas produced is Xenon (Xe).

* Nuclear fusion of Hydrogen: This produces Helium (He) as the primary gas.

To give you a more specific answer, please tell me:

* What elements are involved in the nuclear reaction?

* What type of nuclear reaction is it (fission, fusion, etc.)?