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You play through a selection of battles from a bundle of different Gundam series. It doesn't tell them in full detail, unless that story is pretty short, like some of the OVAs.
All of the stories are from the first half of the Universal Century (Original Gundam to Unicorn). If you want some AU series, like Seed or 00, those will appear in a different game (G Generation Cross Rays) releasing this year.
The Haro stages and stuff like Hot Scramble Gundam are free dlc. They are handy if you want the option to gain resources fast.
And FYI - It is very easy to fail the MS IGLOO finale aka one of the first few stories you can go through. It is because after you beat the challenge you get 3 near dead allies spawned north of you being chased. The 3 challenge units spawned can easily kill them and lose you the mission so, like, be prepared.
None of the dlc is needed but it is nice to have options.