Answer
$\Delta G^{\circ}$ = 0 (at equilibrium) and $\Delta G \ne$ 0 (when the number of H and T coins are not in equal amounts)
Work Step by Step
When both H and T are in equal amounts the reaction does not take place as there is no driving force that proceeds the reaction to either side. In this case, $\Delta G^{\circ}$ = 0.
When the H and T coins are not in equal amounts, the reaction has driving force and $\Delta G \ne$ 0. When reaction is at equilibrium then $\Delta G = $ 0
