You should get tender gnocchi with this recipe if you're using boxed gnocchi and using an ovenproof skillet. Fresh gnocchi has a similar (or slightly shorter) cooking time. If you're transferring the mixture from a non-ovenproof skillet into a baking dish, the cooler temperature of the baking dish might mean it takes a couple more minutes for the gnocchi to be perfectly cooked through. Just test it when the baking time is up and see if it's tender enough.