Sorry for the very long post. But this is question that every Gundam fan has asked forever. Do the Gundam series take place in at the same time. My theory is that they do, or most of them do. All of them but Turn A and Age are in the last and in the past all of the people of earth spread out of earth into other places of the galaxy. An each place has their own Earth and colonies and everything. Now since that happened the other places don’t know why happens in the other places. All of the places are individual. So one places may have Zeon while another place has Celestail Being. Age and Turn A take place in the future because both of them hint that Gundam used to exist. And they both have Dark histories of wars. And both of them abandoned the old technologies. But in both of them they have remnants of the past. That is my theory.
In the Universal Century timeline, “Gundam” is the name of the Earth Federation’s (Chikyū Renpō, 地球連邦) first experimental general-purpose mobile suit, which is incredibly powerful, compared to most of the mass-produced models eventually used by either side. Afterwards, many powerful mobile suits based on the Gundam’s design also carry the name, such as the Gundam Mk-II, Z Gundam, LM312V04 Victory Gundam, etc. The Anaheim Gundams, or Greek alphabet Gundams are a series of mobile suits developed by the fictional company Anaheim Electronics, some of them are notably carrying different style heads than other Gundams in the series, like the Rick Dias (Gamma Gundam) and Hyaku Shiki (Delta Gundam). In this timeline, Gundam often stands for General-purpose Utility, Non Discontinuity Augmentation Maneuvering weapon system.
In the Future Century timeline, the word “Gundam” refers to a space colony’s entry in the “Gundam Fight”, a competition that determines which colony would rule the Earth for the next 4 years. The pilots who use these Gundams are known as “Gundam Fighters”. However, some suits that are called Gundams are not entries in the Gundam Fight at all, with the most operative examples being the JDG-009X (JDG-00X) Devil Gundam and the JMF-1336R (JMF1336R) Rising Gundam.
In the After Colony timeline, the word “Gundam” refers to most mobile suits constructed out of a special alloy called “Gundanium”, which can only be mined and produced in space. This alloy gives the Gundams high resistance to conventional weaponry and renders them stealthy under radar, due to its nature of absorbing electromagnetic radiation. Every Gundam has a unique name that befits the nature of the suit and/or its origins, such as the XXXG-01W Wing Gundam, XXXG-01D Gundam Deathscythe and the XXXG-01S Shenlong Gundam.
The After War timeline is a loose continuation of the Universal Century, presenting an alternate scenario, with many of the mecha designs loosely based on Universal Century suits. The “Gundam” in this show refers to mobile suits from the previous era built from lunar titanium alloy, and suits made in the After War era that were based on Gundams.
In the Correct Century timeline, the name “Gundam” is given to the WD-M01 Turn A Gundam mobile suit by a specific character, who was originally an ace pilot, but was placed under suspended animation. The color scheme of the Turn A reminds the character of the Gundam mobile suits from previous eras.
In the Cosmic Era works, Gundams are mobile suits that carry a specific type of operating system, first secretly produced (without the programmer himself knowing what he is programming the system for) by the one of the protagonist’s robotics research group, and faced a few upgrades by various factions throughout the story. The acronym of the operating system’s original version and its variations always spells “GUNDAM”, hence the name of the suits. However, this is retconned by later toys and scale models tagging the “Gundam”, after the unit’s own name.
In the Anno Domini timeline of Mobile Suit Gundam 00, the term Gundam is the generic designation of a line of unique and highly effective mobile units developed by Celestial Being, such as the GN-001 Gundam Exia and GN-002 Gundam Dynames. Distinct from concurrently introduced weapons, suit torsos contain a core generator system known as the GN Drive or Solar Furnace, capable of sustaining independent unit function for an indefinite period of time without needing to refuel. In addition, onboard propulsion recycles the generator’s GN particle exhaust for high-speed, virtually unlimited output. The particle exhaust disrupts the use of conventional radar and a range of wireless communications.
In the Advanced Generation of Mobile Suit Gundam AGE, the Gundam was a legendary mobile suit developed by ancestors of main character Flit Asuno. When he inherits the AGE Device from his mother, which contains plans of a mobile suit that resembles a painting of the Gundam he saw, he decides to name the new mobile suit (and its following models) after it.
The majority of Gundam, including the earliest series, occur in the Universal Century (UC) calendar, with later series set in alternate calendars or timelines mostly unrelated to the UC system (at least three of these calendar systems were actually initially numbered after the year that the series premiered, with 1979’s Mobile Suit Gundam taking place in UC 0079, 1995’s Mobile Suit Gundam Wing in After Colony 195, and 2007’s Mobile Suit Gundam 00 taking place in 2307).
Mobile Fighter G Gundam takes place in the Future Century (FC), where space colonies have gained the upper hand over Earth, consequently attempting to put an end to the internal power struggles now plaguing the planet.
Mobile Suit Gundam Wing takes place in the After Colony (AC) calendar system, set after the foundation of the first space colony, where Earth firmly maintains repressive control over its colonies.
After War Gundam X takes place in the After War (AW) timeline, set after the conclusion of the 7th Space War, where the Earth is devastated by massive colony drops.
∀ Gundam takes place in the CC (正歴 Seireki?) calendar system, which director Yoshiyuki Tomino intended as the distant future of all previous timelines. The CC sees the colonies becoming much stronger than the Earth, but unlike the FC timeline, takes a more aggressive stance towards it. The English acronym of CC is Correct Century (コレクトセンチュリー), in the official Japanese ∀ guide book.
Mobile Suit Gundam SEED, its sequels, and spinoffs take place in the Cosmic Era, with CE referring to the years after the foundation of the first space colonies.
Mobile Suit Gundam 00, is so far the only series to occur during the non-fictional Anno Domini (西暦 Seireki?) timeline. This is reflected in the show with a worldview that more reflects the modern world, and the presence of real-life conflicts and rebel groups such as the Real Irish Republican Army and the Liberation Tigers of Tamil Eelam.
Mobile Suit Gundam AGE takes place in the Advanced Generation (AG) calendar system, which is set after hundreds of years of space colonization and is characterized by the end of all wars in the Earth sphere and a period of peace before the sudden attack of the UE.
Bandai and Japanese-speaking fans unofficially refer to projects not directly related to the first Gundam series or its staffers (such as Gundam Sentinel and Mobile Fighter G Gundam) as “Another Gundam” stories, and to projects made after 1989 as “Heisei Gundam” stories. On a survey for a video game that would become Gundam: True Odyssey, the Cosmic Era series (including Mobile Suit Gundam SEED Astray) were collectively referred to as “21st Century First Gundam” (a reference to the original Mobile Suit Gundam series). English-speaking fans have used the term “Alternate Universe” (“AU” for short) as a nickname for the stories that do not take place within the Universal Century timeline, but this is made unofficial for use in Japan.
All this info is from Wikipedia’s page for Gundam.