It's called QP for "Quarry Product" and it is unwashed crushed stone, so it's stone and dirt and a lot cheaper than most gravel. If you use gravel, you want crushed stone with lots of edges.

You'll need to build a platform using retaining wall blocks. The platform needs to be pretty wide. Rule of thumb says however high the platform is (say, 1 foot) the distance the pool wall needs to be from the edge is double that--in this case, 2 feet. Us the highest point of the platform. So if your platform is 2 feet high, it needs to be four feet wider than the pool on each side. If that's too big, hire an engineer who knows road building to tell you how you can size it down, safely. I did for my pool and it was worth every nickel of the $500 I paid him.

Then you'll want to put perforated drainage pipe at the base of the inside of the platform, below the wall, and daylight it beyond the platform. You don't REALLY need it with retaining wall blocks and QP, but it can help. Fill with the QP, and then rent a tamping machine and tamp it down--level of course. Leave about 4" for sand and use that for the base of the pool. The QP drains as well as crushed stone, is cheaper, and has a higher friction co-efficient. Check with someone building a development where they are building roads as well as houses. They may well have the QP for sale--probably very cheaply.