Gaia

Gaia (ガイア [gaia] in Japanese) is a recurring world name in Final Fantasies, although it must be noted that for many of them the name is only present in the English version.

Final Fantasy I

Gaia Versions
Final Fantasy I
Stranger of Paradise
Dissidia/Duodecim

Gaia is the world of the very first Final Fantasy, and despite its apparent simplicity, exists in several dimensions, proving that the simplicity is mostly deception. The main dimension is present in Final Fantasy I and Stranger of Paradise, albeit different timelines of it, while another version of the world exists in the Dissidia series. This second version separated from the main world in the distant past due to a dimensional rift - or perhaps was found/created within a dimensional rift, the distinction isn't clear. Civilization exists only in the main version.

As far as can be told, Gaia has one white natural moon. Its main continents (all officially unnamed) are the Onrac continent to the north-west, the Gaia continent to the north-east (shaped like an eagle), and the biggest continent to the south. The last of these has a big inland sea called the Aldean Sea, and also has the most current-day civilization.
The equilibrium of the Gaia world is held together by four crystals, those of Earth, Fire, Water and Wind. The state of the world is immediately reflected in the light emitted by the crystals and vice versa.

Thousands of years in the past, the world of Gaia was controlled by two major civilizations, one who relied on advanced technology (ancestors of the people of Lufenia) and one who relied on summoning magic. The two civilizations were at war, and eventually the Lufenian civilization discovered or created the creature who became known as Chaos. Eventually Chaos used his powers to rend a rift into the fabric of the world and escaped into the Rift, where he settled down into the other dimensional version of Gaia, while the Lufenian civilization was almost completely destroyed and the other civilization languished and regressed. Chaos eventually returned to the original world, where a time loop was in effect for an unknown amount of cycles. This time loop consisted of Chaos ruling the world in the distant past, living through thousands of years and forgetting who he was, becoming an ordinary knight, and getting transported into the past on the brink of death to find his identity and start all over again. The loop was broken by another facet of the loop, the Warriors of Light, who eventually managed to defeat Chaos instead of the other way around, bringing a new future to Gaia where Chaos and the Warriors themselves were mere myth.

Final Fantasy IV

Gaia Versions
Final Fantasy IV
The After Years

Gaia in Final Fantasy IV, sometimes also known as the Blue Planet (and exclusively known as the Blue Planet or 青き星 [aoki hoshi] in Japanese), is the third planet from the sun in its solar system. The other planets in it seem to correspond roughly with the planets in Earth's solar system, up to their Japanese-language names (火星 'fire planet' for Mars, 木星 'wood planet' for Jupiter, etc), with the difference that in ages past a planet existed between Fire Planet and Wood Planet - in the real world, the planet between Mars and Jupiter never managed to form in the first place and left behind the rubble of the asteroid belt. Said planet in the FF4 universe was destroyed, and a remnant of it became a new artificial moon for Gaia, guided by the remnants of the civilization who had lived on said planet. In the real world there are likewise fragments of the would-be planet. The biggest shard, known to us as the dwarf planet Ceres, is smaller than the moon on real-life Earth, and more or less has the right circular shape.

Gaia has two moons, a bigger white, natural moon and a smaller artificial moon - the aforementioned shard of the destroyed planet - that sometimes gets a red tint to it and/or moves closer to the planet as per the navigation system on the moon. Gaia itself has one major continent to the north and two smaller ones to the south-east and south-west respectively, as well as various smaller islands.
It is unknown what role exactly the Crystals have in the upkeep of the world, but they appear to be important, or at the very least powerful. The aboveground of Gaia has 4 crystals and the underground another set of 4. The artificial moon also has a set of 8 crystals total.

The civilization on Gaia itself appears to be only starting out, although this is contradicted by how a certain inventor found instructions in a desert ruins on how to build a primitive flotation system (aka the system that keeps airships afloat). It is unknown what kind of civilization could have had the required skills and knowledge to built it. Current-day technology seems mostly limited to said airships, as well as magic.
The civilization on the destroyed planet-become artificial moon appears to have relied on various crystals for their technology along with magic, as evidenced by how all known constructs of theirs appear to be controlled by a set of crystals.

Final Fantasy VII

Gaia Versions
Final Fantasy VII
Advent Children
Before Crisis
Crisis Core
Dirge of Cerberus

Gaia in Final Fantasy VII is the third planet from the sun in its solar system. Its name comes from promotional material and area names, in in-game text the planet is most often referred to as the Planet (星 [hoshi] or 'star' in Japanese). The other planets in it seem to correspond roughly with the planets in Earth's solar system, although their names are unknown. It appears though, that the world has advanced telescopes available to know so much of their solar neighborhood, and it follows reason that they would be named. Gaia itself has one natural white moon.

Civilization on Gaia is old enough to have gone through at least two major civilizations. The first of these, known as the Cetra, inhabited the planet long ago, and while not much of their civilization remains in the current day, it appears they built their houses to resemble various seashells. They are also known for building temples roughly the same in appearance as ancient Mayans on Earth. Their rule was brought to an end around the time Jenova arrived on the planet.
At some point - it's unknown if this was before or after Jenova brought calamity to the Cetra, settlers from the world of Spira arrived. At least some of their descendants survived, and they might have had children with the Cetra. In the current day, the civilization of the humans has advanced to using oil and coal and beyond, in the last several decades into using mako energy which seems to require fusion engines or something very much like them. Much of the world is industrialized or being exploited for the industrialized world, although this exploitation appears to stop at the end of FF7.

Final Fantasy IX

Gaia Versions
Final Fantasy IX

Gaia in Final Fantasy IX is the main world of the game's events. It is a relatively young one whose planetary crystal shines bright blue. Natively, Gaia has one blue moon, although due to an attempted merger of Gaia and an older planet called Terra, a slightly smaller red moon (Terra's moon) was also visible in the skies before the end of the game when the world merger was stopped.
The moon cycles during the fusion phase of Gaia and Terra are as listed below.1 The two moons have highly elliptical orbits that do not line up, ensuring that twice in each moon cycle one moon is in front of the other. The length of a single moon cycle is 64 days, well over double the length of a moon cycle in the Earth of the real world.

Day	Gaia's Moon	Terra's Moon
1	In front of Terra's Moon (close to Gaia)	Behind Gaia's moon (far from Gaia, not visible)
16	Behind Terra's moon (far from Gaia, visible as an outer ring)	In front of Gaia's moon (close to Gaia)
32	In front of Terra's moon (close to Gaia)	Behind Gaia's moon (far from Gaia, not visible)
48	Behind Terra's moon (far from Gaia, visible as an outer ring)	In front of Gaia's moon (close to Gaia)

Gaia has three main continents, one lesser one, and various islands. Current-day civilization is concentrated on the Mist continent in south-east. It received its name due to the prevalence of Mist, a by-product of the Iifa Tree that binds the two planetary cores together and exchanges souls between them. To the north of it is the Outer continent. In the present day only goblins and black mages live there, but only a dozen years or so in the past it was home to the summoner race. To the west of the Mist continent lies the Forgotten continent. No native habitation is known there, although remnants of the Terran civilization have transferred there due to the attempted merger. Lastly, the smallest continent is the Lost continent to the north-west, home to mostly ice, snow, and a single volcano.

The people of Gaia appear to be a varying bunch. What is considered simply 'people' covers a wide variety of features, such as antropomorphised hippopotamus and other mammals and various different birds, in addition to the more human-looking residents. In fact, in all the variety of 'people', only two different native races from that seem to be recognized, those of the rat-people (or Burmecians) and the aforementioned summoners. The level of technology varies greatly by country, with the most advanced country, Lindblum, having invented both steam engines and Mist engines.