Here is what Gibbon actually wrote:

After a diligent inquiry, I can discern four principal causes of the ruin of Rome, which continued to operate in a period of more than a thousand years. I. The injuries of time and nature. II. The hostile attacks of the Barbarians and Christians. III. The use and abuse of the materials. And, IV. The domestic quarrels of the Romans.

Not sure what else there is to say.

Quick edit: I did think of something else to say: Gibbon is a nice read but isn't a proper source for actual historical information. To put it mildly, his work does not meet modern academic standards.

There are so many problems with this I almost don't know where to start.

Firstly, Gibbon's views are no longer widely considered credible. He writes as an 18th historian with 18th century biases, and while it's right to engage with his views, they often do not hold up under contemporary analysis. In particular, the causes of the 'fall' of the Empire are a matter of endless debate, and even the idea of a 'fall' of the Roman Empire is subject to more critical examination.

Secondly, point 4 is not very accurate, because the nature of military expenditure is very different. point 5 is wildly inaccurate, because Gibbon was very critical of religion, particularly Christianity, and considered Christianity one of the factors of the fall of the Empire.

Thirdly, there are significant issues of comparability here. Is America an Empire? On balance, I'd say yes, but the socio-political reality of the 21st century is so vastly different than 5th century Rome that trying to draw abstract generalisations and comparisons is almost useless. I consider this an instance of the way the 'idea of empire', and specifically the ideology of Rome, continues to shape Western culture(s), and American civic ideology. This is getting away from history proper, and into contemporary studies, so I will leave it there, apart from asking what the rhetorical value of comparing America to Rome is - isn't that the critical question to ask of these types of comparisons?

It's important to remember that while Gibbon was writing one of the first works that is recognisably 'modern' history, he was also writing something of a polemic about the British empire as it existed at the time.