Sure an apple seen falling from a tree is evidence of gravity, but like wind, gravity is not necessarily something that can be truly seen.
NASA has published an image though that can give you an idea at least of what a "gravity wave" looks like.
Atmospheric and surface phenomena observed in this photograph taken from the ISS last month. (Photo: NASA)
The image taken by an astronaut from the International Space Station above Lake Superior on June 24 has "gravity waves," those parallel bands in the clouds.
"Gravity waves are produced when moisture-laden air encounters imbalances in air density, such as might be expected when cool air flows over warmer air," NASA stated. "This can cause the flowing air to oscillate up and down as it moves, causing clouds to condense as the air rises and cools and to evaporate away as the air sinks and warms. This produces parallel bands of clouds oriented perpendicular to the wind direction. The orientation of the cloud bands in this image, parallel to the coastlines, suggests that air flowing off of the land surfaces to the north is interacting with moist, stable air over the lake surface, creating gravity waves."
The photo shows not only gravity waves but another atmospheric phenomenon called "sunglint."
This latter phenomenon is a reflection of light on the water surface "resulting in a bright, mirror-like appearance over large expanses of water."
"Water currents and changes in surface tension—typically caused by presence of oils or surfactants—alter the reflective properties of the water and can be highlighted by sunglint," NASA's description continued. "For example, surface water currents are visible to the east of Isle Royale that are oriented similarly to the gravity waves, suggesting that they too are the product of winds moving off of the land surface."