The main difference in the gundam series can be summed up in UC vs AU.
UC series: MSG,0080, 08th Ms team, 0083, Zeta, Double Zeta, Char's Counterattack, F-91, Victory. They all share common bonds and themes and are based in the same universe.
AU series are series that were created to rejuvenate the gundam franchise after the commercial failure of Victory.
G gundam is the black sheep of the family. It drops the war theme and and goes with a tournament style battle system to see which country will rule the universe for 4 years. It features the gundams performing stunts similar to Street fighter.
Wing is the most well known of the gundam series outside of Japan. No need to go on further about that.
X haven't seen it, but I've heard of it described as a what if scenario of the colony drop from MSG and deals with the aftermath of multiple colony drops.
Turn A. also haven't seen this.
Probably the most unknown gundam series there is.
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