Nothing will alert them unless you are upwind. And then anything will. Not trying to give you a smart answer. It's just that nothing fools their noses.
Anything with a high percentage of DEET, such as DEEP WOODS OFF and CUTTERS work pretty good but make sure you apply a couple of times throughout the day if you are sweating or rubbing off the areas you sprayed. REPEL Sportsman's Max works pretty good as well with 40% DEET and you can pick that up at Home Depot I think. Those gnats are pretty mean and you need to make sure you stay covered, but DEET 20% and above will keep them from biting you.
BB