The book is a historical novel first and foremost. And it sacrifices historical fact for fictional convenience. One small example is the location of 20 Maine on the third day. Shaara placed them just behind the center of Cemetery Ridge so that the primary "hero" on the Union side, Chamberlain, can narrate Pickett's charge for us. In reality they were about a mile further back and unable to see the fighting.

Shaara also picked the characters he did for narrative purposes. He was explicitly highlighting the brother vs brother and lost cause themes. And it isn't an entirely accurate portrayal of several of the characters. Chamberlain, Lee, and Longstreet for example are all described in almost hagiographic language.

And finally some facts and characters are entirely fictional. Kilrain for example is Shaara's narrative voice character. And as such he tends to wrap things up for us in neat packages and explain things to us in 20th century terms.

All that said it is good historical fiction. It is well written, and well researched. It contains a lot of historical fact. And it includes a ton of actual direct quotes from the principal characters. But it is still fiction and doesn't worry about tossing out facts to improve the narrative or the themes the book is based around.