I guess you have your answer already gravel is not really a good idea. If you were going to do a bio-active substrate it could be the base layer but it is a balancing act and requires lots of research and a fair bit of experience to pull off.

There are lots of good workable solutions out there, a plethora of paper products, I'd never recommend feeding on paper towel however...

A bunch of wood chip products. NO CEDER! Pine is questionable I'd avoid that too. As well as some coco products (chips and coir) As well as a few synthetic options. Each has advantages and dis-advantages but all of them will work.