I'm gonna be a fly in the ointment. I agree with Old hen. The gizzard is designed to function with grit. The digestive system is designed to function with grit in the gizzard. While the crop sits in front of the plumbing system, it makes sense to me that an impacted crop is symptomatic of a digestive system that is not functioning properly. So, yeah, grit in a gizzard is not going to "fix" a crop impaction. But, perhaps if the gizzard had plenty of grit, the impaction would not have happened in the first place.
However, I do not believe that grit is the be all and end all of preventing crop impaction. To my knowledge, my birds have never had crop impaction, in spite of my laissez faire approach regarding allowing access to hay, and long stringy heaps of garden debris in my coop and run. (one of the "they say" things that causes crop impaction) I'm thinking that an active gut with healthy gut flora is a big part of the impacted crop prevention. Lots of access to deep composting litter, and fermented food help to keep the gut flora balanced.