Edward Gibbon does not concern himself much with the republican era, when the secessio plebis occurred. As his title foretells, he concerns himself mostly with the Decline and Fall of the Roman Empire (not its rise). Thus he begins after the five good emperors, i.e. Commodus, as a pinpoint from whence the decline begins in earnest.

He reflects on earlier terrible emperors, like Nero and Caligula, and contends that even though terrible, they were nowhere near as terrible as Commodus, nor did their behaviour have lasting influence on later emperors, hence the five good emperors following them.

Gibbon suffers in perspective from the era he was writing in, late Renaissance Enlightenment era, where blaming the church was a recurring theme. He lays heavily blame on Christianity for the collapse of the Roman Empire, and downplays the church's official perspective on the persecution of Christians before the Empire became Christian.

While I can heavily recommend his work, I also strongly advise people to view Gibbon through the lens of when he was writing. Gibbon's usage of original sources and footnotes makes him a founder of modern historical sciences. And in that regard, his work is incredible. But as another commenter said, we have learnt a lot about ancient Rome since the 1770s.