According to Hesiod, Gaia was the first being to be born of Chaos. She was then followed by her siblings Erebus, Nyx, Eros, and Tartarus. Apparently born pregnant, Gaia gave birth to Uranus, Ourea (the mountains), and Pontus (the sea) parthenogenetically (without the help of a male). Surrounded by the sea, Gaia may best be thought of as a large, pregnant belly, upon which we dwell.
Together with her son Uranus, she bore the Titans, the Cyclopes, and the Hecatoncheires. With Gaia's help, Uranus was overthrown by Cronus, and Cronus was in turn overthrown by Zeus. In this way, Gaia can be seen as the guiding force behind the gods.
With Zeus in power, Gaia joined with Tartarus and gave birth to her final son, Typhaon, who had 100 snake heads writhing up from his shoulders. He nearly overthrew Zeus, before finally being beaten.
Gaia had other children as well. With Pontus, she gave birth to Nereus, Thaumus, Phorcys and Ceto.