I always thought as the place in Oeilvert as some kind of "Gaia museum" i mean basically it shows the evolution of Gaia and it´s development, i think that´s what those giant faces talk about also.

As for the Gulug stone it´s connected to Mount Gulug...(it´s only a suposition) because i don´t think they actually need it to access Terra.

The same thing can be asked about Ipsons castle, were the four mirrors are located, it kind of serves as a map to reach Terra.

Oeilvert...
I honestly thought that that place was some sort of memorial to the Terran civilization, not to mention something akin to a grand archive.
The stone faces actually explained Terra's recent history (by recent, I mean the last few decades, or centuries, before the parisitic plan for Terra's revival was conceived). The entire place chronicled Terran achievements through the ages.

Anyway, they just mention that Terra has absorbed other planets before; they never mention how many or give any names. All we really know is that Gaia would be the latest victim if the plan went through.

Short version to Aztec's answer: "No one knows"
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Long version below with tons of spoilers. (Freak'n thing took me close to 2 hours to put together...)
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Well the first mention of Terra doesn't come until the party reaches Oeilvert, which Kuja wishes Zidane and party to go there to retrieve the Gulug Stone. Kuja doesn't go because:

As they enter the Planetarium they see a glowy thing with an image of a planet, and some inscription that Zidane states is talking to him. He murmurs something about "Terra" and "Mother". He's not really sure what it all means though.

It then goes in to telling him about an ancient ship that was used to wreck havoc in war and such and it came to be known as the Invincible. Then a hologramish looking image appears and Zidane reads it aloud:

Zidane: Beginning...of the city...
(Zidane): ...?
Zidane: Huh? Now it's gone again.
He taps it, and a new hologram appears.
(Zidane): Whoa!
Zidane: At first...the city...flourished...
More holograms.
Zidane: Soon...began to decline...
A hologram of a planet in peril.
Zidane: At its peak...cities...all over...Terra...
Zidane: Then...the decline...began...

When the party gets to the stone they are instructed to climb ontop of it and faces start to appear and they start to inform teh party of the history of Terra.

In the Face Room:
A face glows.
Visitors... Climb atop the stone...
Visitors, listen well...
This is the record of our civilization...
Another face.
The seeds' decline was not our fault...
All vegetation and...vessels...died out...
The decay of our Terra... triggered it all...
In order to stop the decay, many methods were considered...
Another face.
Ultimately, the best minds of Terra's civilization gathered...
Then, a final resuscitation was attempted on the mother continent...
Unfortunately, it ended in failure...
It was later discovered to have been the catalyst... Subjects were selected...
Another face.
After four tremendous sacrifices, eternal prosperity seemed within reach...
Flora and fauna were revived, but...is still in stasis.
Final results...are still pending...
Another face.
In order that our glorious history not die with us, we were constructed...

In the Star Room:
They then grab the stone and head back to Kuja. Kuja though in the midst of this knows of Garlands plan to have Zidane suceed Kuja in the mission. Zidane then wants the Eidolons extracted from Garnet so he can find one that he can bury Garland with; one more powerful than Alexander even.

Remember that Kuja was with Hilda (Cid's wife) for a while and he revealed amny things to her. Hilda mentions that Kuja is not from Gaia at all but instead is from Terra.
Hilda then talks about how to get to Terra from Gaia. Remember the Shimmering Island? Apparently they are connected, but you can't go directly from Shimmering Island to Terra. The gateway that connects to two worlds is sealed and can only be broken in Ipsen's Caste. (Cool fact too: Hilda names it Ipsen's castle. The official name is never mentioned, so she sorta nicknames the castle that).

Once they get to the castle Zidane reads some more Holographic mirrors where it says 4 riddles and hints of where the seals; hence the 4 element guardians you then face. A quick boss interupts the party though here demanding to have the mirrors returned. You kick the boss' ass, and you go on your way to face the 4 guardians.

However, once the members are at the locations, each of the guardians begins to talk to them all:

Once the Party then gets to Terra Zidane starts to realize that everyone looks like him. They ahve no age, they don't need sleep, and one mentions that when "the blue light turns red, that is when the souls enter their bodies. (Terra being the Red planet and Gaia the Blue)

In theMeeting Hall:

Loner: A planet's shimmer is its breath, a sign of life-activity caused by the cycling of souls.

* (1): When the souls of Terra and Gaia mix, Terra will be integrated into the larger Gaia...

* 2/3
(3): Souls flow...
(2): As the planets turn...

* (4): Fleeting knowledge is useless to vessels... But information is given to prevent mental atrophy.

* (5): A planet's color indicated its shape... Gaia is blue, Terra red.

* (6): The flow of souls means that a planet has life...
(6): The stoppage of souls means that a planet is dead...

Here's one of the best parts of the game IMO:
Zidane: I'm alive! I have a soul! Why is everyone here so...
Girl: Because you are...special.
(Zidane): What!?
Girl: We are mere vessels. You have been given a greater purpose.
Girl: The will of Garland is absolute...
Zidane: Garland? Who's that?

Back in the Meeting Hall again:

Girl: Garland watches over this planet. His mission is to restore the people of Terra.
Zidane: R-Restore the people of Terra? You mean people besides these guys here?
Girl: They are merely vessels. So am I, and so are you.
Girl: But the true people of Terra have been asleep for ages, waiting for their time to come.
Girl: When the time is right, the light of this planet will change from blue to red, and Gaia will become Terra.
Zidane: Wait a sec! So the people of Terra are out to take over Gaia!?
Girl: What else can there be? Terra has always absorbed new planets to survive.
Girl: And when the time comes, the souls of people of Terra will occupy the Genomes. Such is the restoration of the people of Terra.
Zidane: Why do they have to be so tedious!?
Zidane: Why don't they just wipe us out with magic like they did Alexandria!?
Girl: Once, Garland tried to use a more forceful method, but failed.
Zidane: Then he should just quit and leave Gaia alone!
Girl: Because of his failure, he was forced to wait a long time.
Girl: ...To wait until Gaia's civilizations gained tremendous power: The power to use eidolons.
Zidane: So that's why you attacked the summoner's village and destroyed Alexandria.
Girl: Planets have a cycle of souls. Souls are born from the planet, and then return to it.
Girl: Garland planned to gain control of that very cycle...
Girl: ...and you were supposed to help him achieve that.
(Zidane): What!?
Girl: Garland is waiting for you. Come...
Zidane: What do you think you're doing!? I don't care about all this Terra and Gaia stuff!

Then Zidane goes through the whole depression mood as everyone tries to talk sense into him. He faces Garland and tells him off, and then Kuja comes in on teh Invincible and blows a fatal blast to Garland. Garland then reminds Kuja that Zidane will grow stronger and Kuja will die. Of course then Kuja goes ape-shit and destroys all of Terra and almost everyone is left for dead.

I heard from a guy that readed the FFIX UOG, that each planet have a crystal in its core. Terra's crystal was dying, so Garland begun to "suck" the life of other crystals of diferent planets, but it don't state how many planets he destroyed.
Then he found Gaia, that have a very young crystal. And Garland begun his plan with the Iifa Tree(a LONG time before the begin of FFIX). But after some time, Garland tought the process was too slow(or something like that, i don't remember very well) and tried to make it more fast. He tried to fuse Gaia and Terra with each other(spiritually AND physically) with a spell named "Fusion". But it was a disaster. Millions of people in Gaia died, and many of them had their bodies fused with animals(this is why we have human/animal people in the game). With this fusion between the planets, Terra was "absorbed" by Gaia, and is located in the core of it. Many places that were originally in Terra(like Oilvert), were fused with Gaia, becoming part of it.
All of these thing about "Fusion between the planets" is what Mikoto was talking about when she said something like "Long time ago, Garland tried to absorb this world by force, but failed". And the two planets "colliding" that we see in Memoria, are Gaia and Terra being afected by Garland's "Fusion" spell. And if i remember well, is possible to see Gaia's crystal in Terra(because it is located in the core of Gaia), and the roots of the Iifa Tree(the first time you go to Terra, before reaching the city) that connect the core(Terra) with the surface(Gaia).

This is not very well explained, because i don't remember all the details of what the guy said, but it was practically that.