POOL. FILTER. SAND.
$10 / 50 lb. bag at hardware stores (Home Depot, Lowes, etc).

Gravel sucks to keep clean, it's not great for fish to dig in, it looks unnatural (yes, even brown) and it's not ideal for growing plants. Whether you want to go dirt tank is a whole debate, but if you decide you wanted a planted tank and went sand (with no dirt) you just shove in a bunch of root tabs, it's still ideal for roots to spread through.

There are two things Petco / Petsmart do to ruin new fish keepers - the first is selling them overly small tanks (<29 gallons) and the second is selling them a substrate that sucks for growing plants.

People INEVITABLY decide they want to try plants, and that they want more fish, and then suddenly their 5-gallon filled with fruity pebbles is causing them frustration.