Whether a "chemical" treatment like the Steredent/Polident will work depends somewhat on what the flask material is and on what the gunk is that wants removing. It's probably safe enough to try.

A minor disadvantage of the acquarium gravel is that it tends to have sharp corners. Scratches produced by using it could slightly increase the rate at which new gunk accumulates.

An "old maid's method" is to put rice (uncooked) in, with sufficient liquid to let it slosh, and then shake. You can continue to use the same rice while draining off the liquid and replenishing. When the liquid that you decant off is clear, you probably have removed about as much as you're going to get.

This is a "gentle" abrasive, so it takes a lot of shaking for complete cleaning. With a proper balance of rice and liquid a nice "shaker" sound can be produced, so you could "lend" it to a kid at a festival and tell them it's an "instrument." If the sound is even slightly annoying, the kid will probably do a fairly thorough job by the time you recover it a the end of the fest - which is far more than you yourself are likely to have the patience for.

John