When you stop to think about it, our planet does act like a huge organism. If you look at the interrelationship between plants and atmospherics, animals and humans, rocks and water, a complex pattern of symbiotic processes seem to complement each other perfectly. Should one system be pushed out of balance by some external force (such as a massive injection of atmospheric carbon dioxide after a volcanic event), other processes are stimulated to counteract the instability (more phytoplankton appear in the oceans to absorb the carbon dioxide in the water). Many of these processes could be interpreted as a "global immune system".
Could Gaia be sentient in some unrecognized sense? If so, how might it communicate (assuming it wanted to)? In David Brin's "Earth," Gaia achieves self-awareness via the electronic nervous system we call the Internet, but perhaps it doesn't need anything so fragile or human-friendly. A global consciousness might manifest in the planet's ambient EM field, in its chemicals, in the molecular architecture of its organisms.