In the field of Geophysical Fluid Dynamics it is an important distinction as there are other very important waves. Rossby waves are not gravity waves and extremely important to the global climate (see their role in ENSO dynamics). Compressive waves (acoustic waves) are everywhere of course. There are also topographic Rossby waves, internal waves and Kelvin waves (note: kelvin waves and internal waves are gravity waves as well). Oh, and inertial waves!