Electrolysis is decently efficient (>70%), but turning the Hydrogen back to electricity loses quite a bit. Nevertheless, the real question is cost, not efficiency. As you noted during peak production the electricity is essentially free. If it's cheaper to use existing NG infrastructure to store Hydrogen and run gas turbines with it than building an equivalent amount of batteries, then we should go for it. Given that world lithium battery production is insufficient right now, and different battery chemistries are still experimental, proven technology like electrolysis+gas turbines seems like a good idea.