There are at least two forms of the Gaia hypothesis:

      The Weak Gaia Hypothesis:
         The ecosphere of the earth has a suite of feedback
         mechanisms which tend to result in some degree of
         homeostasis.  I.e., these feedback mechanisms tend
         to keep the global mean temperature, concentrations
         of various gases, etc. at more or less constant
         levels.  These feedback mechanisms cause the
         ecosphere to act in ways analagous to the functioning
         of living organisms.

      The Strong Gaia Hypothesis:
         The earth is a living organism.