Now, I'm dumb as........ but this seems to be a sensible explanation I can understand:
"In fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two media (e.g., the atmosphere and the ocean) which has the restoring force of gravity or buoyancy.
When a fluid element is displaced on an interface or internally to a region with a different density, gravity tries to restore the parcel toward equilibrium resulting in an oscillation about the equilibrium state or wave orbit".
My bold. Symplistically, gravity waves must coexist with ripples or waves on the sea surface or in the atmosphere.
Learn something every day.
