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.