I completely agree with the first part of Starline's advice.
I agree with the second part only if you're running a large, commercial boarding establishment.
If you can allow for a couple of weeks of good weather to leave the barn open, and can truly saturate the gravel dust and allow it to sit, it will harden up nicely, even without the commerical tamper. If you only have a few days, go with the commercial tamper. Stone dust packed down correctly will last for 1 - 2 years with adequate bedding and good, thorough, daily stall mucking for horses that are in 12 - 14 hours.
Correctly packed stone dust with mats over it will last for longer. If you have horses up for more than 10 - 12 hours per day, you will run into all the problems that Starline describes, and will periodically have to undergo the disgusting task of lifting the mates, shoveling out the urine saturated stone dust and starting over.