gravitate

from The American Heritage® Dictionary of the English Language, 5th Edition.

• intransitive verb To move in response to the force of gravity.
• intransitive verb To move downward.
• intransitive verb To be attracted by or as if by an irresistible force.

from The Century Dictionary.

• To be affected by gravitation; yield to the force of gravity; tend toward the lowest level attainable, as a rock loosened from a mountain.
• Hence To be strongly attracted; have a natural tendency toward a certain point or object.
• To allow to fall or move downward under the action of gravitation; manipulate (as gravel, in diamond-mining) so that the heavier portions sink to the bottom.

from the GNU version of the Collaborative International Dictionary of English.

• intransitive verb To obey the law of gravitation; to exert a force or pressure, or tend to move, under the influence of gravitation; to tend in any direction or toward any object.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• verb intransitive To move under the force of gravity.
• verb intransitive (figuratively) To tend or drift towards someone or something, as though being pulled by gravity.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• verb move toward
• verb move due to the pull of gravitation
• verb be attracted to