A substrate is better for shrimp IMO as it gives them more to dig through for food. If it is a thin layer you should not have a problem with gas as it will be pretty consistently moved around by the shrimp.
You need a relatively thick layer generally - multiple inches, and the right conditions for hydrogen sulfide to form in an aquarium. Most of the time that people think they have hydrogen sulfide bubbles form in their substrate - its just nitrogen/co2 gas, possibly methane in a really dirty tank. It still smells like rotten eggs because the gas was made from decaying poo. I keep 1-3 inches of sand in most of my tanks with no issues. The shrimp will like a bit of stuff to dig around on.