There is a spectrum of Gaia hypotheses, with a very wide range. One end says that organisms have greatly altered the earth in which they live.  An even more intense theory is the earth is a organism unto itself and it has its own functions including  a way to keep itself balanced and teeming with its own life, and balanced enough that the life will flourish. Scientists  will often simply assume this to be natural evolution, that individual animals or organisms are the cause of the balance itself, through thier own concerns and not that of the good for all organisms. These indivdual reactions somehow give way to balance.

Even more potent a claim is that we are all a part of a single being.  With this idea, everything we know as planetary, or within the planet, such as us, and life including plants and animals, are all the result of Gaia directly, the ever changing of a single organism. While most scientists refuse this idea, some embrace it, and it is thought to be a plausible happenchance.

By far the most extreme theory is that the Earth is a single entity, that the environment is actively working to make life better for its population. There has been zero evidence to actually support this. An understading of homeostasis shoots down this theory.