The name is actually comes from misspelling the word "Gargantua" but don't let it fool you, we are talking about creatures of cosmic scale. All the planets, stars, and other cosmic objects in the universe are either eggs or what's been left of eggs. The theory is that there are seeds roaming the universe and those seeds pull the mass around them to form shells and become eggs for those seeds. In fact, the research shows that there would be nothing bigger than atoms in the universe without those seeds since they are the source of gravitational pull. So the seed eats up the eggs, might consist of gas, plasma or rock it doesn't matter, from inside out to grow, similar to an animal that grows inside an egg. When it is grown enough, it just cracks open what remains of the shell and push itself out of the universe, creating a black-hole. If the creature is one of the big ones and had taken residence in a gigantic star then the black-hole becomes stable and stays there. If it is a planet-Gargatia then the black-hole just closes on itself before being able to pull anything, leaving almost no trace except the asteroids or nebulae left of the shell. An that is how the asteroid belts around star systems form.
There are types of Gargatias as many as astronomical bodies in the universe. Gas-Gargatias reside in gas giants like Jupiter, star-Gargatias - obviously - in stars and so on. Even those types can be sub-typed but since nobody observed any actual creature for more than a couple picoseconds (10^-12 , trillionth of a second), even that happened only twice, it is impossible to predict if those are different creatures of just different in size. However more information is expected to be gathered since they discovered the first creature after building a cosmic camera of ability to capture more than 10^20 frames per second and they already started to build an even more advanced one.
Scientists explained the public how they would be safe after the shock of the discovery. The predictions show that the creature in earth would be thrown out before fully grew. Simple reasoning is that when our sun becomes a red dwarf by expanding significantly, it would eat up the earth and when the creature be born, it would also pull out the Gargatia of The Earth. It will happen in about 5.4 to 6 billion years so humans are fairly safe. As for premature birth, the researchers insist they would see the signs before it would happen, even if it is possible. After all, no one has observed a premature event in any star system that we gazed upon. It helped the public to keep calm but beside that, is it an impossible event or just not observed yet, that is another question.
There is furthermore, the case of seed being dead or dying. There is no real proof of possibility or of an example as the creature's effects on magnetism, life, and other attributes of a planet not yet known. Nevertheless, the voices to say Mars might be an example are getting stronger and stronger. Why Mars lost its water, and maybe even life along with inconsistencies of its magnetic properties, never explained completely and properly. So it might have something to do with a dead seed inside, according to the theory. There might be other examples of planets or even stars but it cannot be proved without understanding the creature's effects on the surrounding "shell" and its habitants.
Another property is that according to the calculations, the seed should in perfect spherical shape. The size is not understood as there may be different types with different sizes. No seed has been observed and it would be almost impossible to see one without the shell since they have an immense gravitational pull, so they just pull everything around them as they roam through galaxies and quickly form shells.
Now the debate is about whether the universe is a womb of another creature and if so, what lies beyond this universe? Can humankind get out of this universe through a black-hole or follow a creature? Maybe these creatures even have something to do with life on earth? Maybe humans can tear apart a small planet and examine one of these before it goes out?
EDIT: Added the changes pointed out in first comment.