Iron blooded orphans is a spin off and very good. Would recommend starting with that as you dont really need to know the story as they explain it pretty well
This, I can agree you can give IBO a shot. I saw the first episode in English dub, which was not bad persay, but I was more focused at the time to watch The Origin manga adapted into OVA movie form which is still ongoing.
I want to start watching Gundam but idk where to start. There's like a kiljillion series
The kiljillion series is due to different spin offs that are often standalone and take place in an alternate universe. Good thing though is Iron Blooded Orphans is standalone and takes place in it's own AU. Which I have heard good things all around about MSG: IBO. You can safely watch it to understand the plot and context, just know it is not connected to the Universal Century timeline (UC)
Gundam Unicorn on the other hand, is the last main series in the Universal Century timeline and universe. The UC timeline is the bread-and-butter MS Gundam series', from where it started, to tropes and themes, that inspire and paid homage to this day in more modern Gundam series airing and already finished.
For best context it is recommended to watch the following: Mobile Suit Gundam, Mobile Suit Zeta Gundam, Mobile Suit Gundam ZZ, and Mobile Suit Gundam: Char's Counterattack.
Where MSG '79 (1979) is available as a series or compilation movies that nail the overall plot well while only losing minor extra, Zeta Gundam movies are usually not recommended because honestly the movie 'translation' makes drastic change to how it ends, which the ending is considered non-canon. It would make the follow-up series not applicable. ZZ Gundam is only available as a series and in Japanese only because for whatever reason it had no English dub done. MSG: Char's Counterattack is a finale movie for the original '79 series with better animation. Of course you could just look up the basic plot and there is also this handy chart for pros vs cons and where to start. Courtesy of /m/
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Tl;dr though, I would recommend trying MSG: Iron Blooded Orphans if you are pressed for time otherwise. Toonami playing a modern Gundam series is a boon for fans and the animation channel block alike. I hope IBO is not replaced for good by Gundam Unicorn, which they should be doing a marathon of early UC so people know more than jack all while giving Unicorn a try.
Really, just because the dated animation did not do well on first run on Toonami with the original MSG series, if they are airing Unicorn they ought to know people need background context one way or another. And I hope it airs alongside IBO's current season or will swap places with IBO once the next season is dubbed.