This is where Gallo becomes a leader instead of a mere fighter pilot. There may be what seems like plot holes in the story but they are intended.
It's 88 pages long.
Don't know where to begin?
While ultimately it doesn't matter which arc you begin with, Cecil arc is recommended if you want my recommendation.
And then Masu arc + Shattered union arc.
Feel free to visit the lore section as well which holds stories of its own.