$\mathrm{NaOH}$ is produced in Nelson cell using Brine solution.

Reaction taking place will be

Cathode: $2{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2+2{\mathrm{OH}}^{-}$

Anode: $2{\mathrm{Cl}}^{-}\to {\mathrm{Cl}}_2+2{\mathrm{e}}^{-}$

In solution ${\mathrm{Na}}^{+}+{\mathrm{OH}}^{-}\to \mathrm{NaOH}$

So, ${\mathrm{H}}_2$ gas is released at cathode during preparation of $\mathrm{NaOH}$.