It's really up to you in many ways, so long as you're clear that you're not reading it for an up-to-date view on the history it's talking about. The only thing I'd not recommend is to try and, say, mentally edit it to make it correct, because at that point you're better off reading more recent works on the subject.

My suggestion -- and this is my bias as someone who specialises in history of ideas showing through -- would be to treat it as a work that reflects Gibbon's objectives, his social and political thought, the development of historiography up to that point, and the intellectual context of his time. There is a fantastic (but very long) work by J. G. A. Pocock called Barbarism and Religion which does just this for Gibbon and Decline and Fall, in great detail, so you may want to check out that if you're interested. The last volume came out just last year.

You can certainly think about a work in a contextual way without having to go back to secondary literature, though. It's a matter of thinking about the ways in which Gibbon's writing is different from how we think about history in the present day. If you're interested in methodology, I would recommend checking out the very influential article by Quentin Skinner, Meaning and Understanding in the History of Ideas (no special access required), which might give you some food for thought.

That's just one way of looking at it, though. You can certainly read it for its aesthetic value, or for its more universal political insights, or even just as a good story. Much older historians like Herodotus, Thucydides, Livy, Sima Qian, and so on, can and are regularly read like that, and it's perfectly legitimate to do so. It depends on what you're looking to get out of it.