This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

n. Movement of a motile organism using the earth's gravity for orientation.

n. A living creature's ability to move by orienting itself by gravitational forces.

n. The orientation or locomotion of cells or of organisms in relation to the direction of the earth (or to gravity).

"Flies are exercised by a mechanized platform, known as the Power Tower, that rapidly, repeatedly, induces their innate instinct for negative geotaxis."

phototaxis   

Axis    axis