The gnats I get are harmless and inconsequential.

Playground sand may well get rid of the gnats in the pots, but there's an army more of them (and their eggs) in the soil bag they came from.

Plus, not at all a good idea to suffocate the roots in an atttempt to control a (virtually) harmless critter. The layer of sand will hold moisture too good, (keeps soil too moist for too long) and restricts the airflow the roots require.

I'm pretty sure it says on the No Pest Strips package to give it a week or so to start seeing results.