Edward Gibbon praises Leibniz as “a bold and original spirit” in his “Antiquities of the House of Brunswick”, in Miscellaneous Works, Vol III, 1796, p. 402. This is mentioned by Lewis W. Spitz in his helpful essay “The Significance of Leibniz for Historiography” (Journal of the History of Ideas, Vol 13 No 3, 1952, pp. 333-48), who mentions the impact of Leibniz on Gibbon and quotes a few lines from Gibbon’s assessment. I went back to Gibbon’s text and there is a very full discussion which goes on for some pages, and is mainly of the detail Gibbon finds useful in Leibniz’s historical work. But there is a long passage on Leibniz as a whole which is interesting because of Gibbon’s admiration of Leibniz’s breadth, but also critical of what this means for his lack of depth (pp. 400-1):
The genius and studies of Leibnitz have ranked his name with the first philosophic names of his age and country; but his reputation, perhaps, would be more pure and permanent, if he had not ambitiously grasped the whole circuit of human science…
There is a discussion of him as a theologian, as a philosopher, and as an a metaphysician. But then, for me, it gets more interesting:
He was a Physician, in the large and genuine sense of the word: like his brethren, he amused himself with creating a globe; and his Protogaea, or Primitive Earth, has not be useless to the last hypothesis of Buffon, which prefers the agency of fire to that of water. I am not worthy to praise the Mathematician: but his name is mingled in all the problems and discoveries of the times; the masters of the art were his rivals or disciplines; and if he borrowed from Sir Isaac Newton the sublime method of fluxions, Leibnitz was at least the Prometheus who imparted to mankind the sacred fire which he had stolen from the gods. His curiosity extended to every branch of chemistry, mechanics, and the arts; and the thirst of knowledge was always accompanied with the spirit of improvement. The vigour of his youth had been exercised in the schools of jurisprudence; and while he taught, he aspired to reform, the laws of nature and nations, of Rome and Germany. The annals of Brunswick, of the empire, of the ancient and modern world, were present to the mind of the Historian; and he could turn from the solution of a problem, to the dusty parchments and barbarous style of the records of the middle age. His genius was more nobly directed to investigate the origin of languages and nations; nor could he assume the character of a Grammarian, without forming the project of an universal idiom and alphabet. These various studies were often interrupted by the occasional politics of the times; and his pen was always ready in the cause of the Princes and patrons to whose service he was attached: many hours were consumed in a learned correspondence with all Europe: and the Philosopher amused his leisure in the composition of French and Latin poetry. Such an example may display the extent and powers of the human understanding, but even his powers were dissipated by the multiplicity of his pursuits. He attempted more than he could finish; he designed more than he could execute: his imagination was too easily satisfied with a bold and rapid glance on the subject which he was impatient to leave; and Leibnitz may be compared to those heroes, whose empire has been lost in the ambition of universal conquest.
The bit about Leibniz ‘borrowing’ from Newton in terms of calculus is now discredited, with the consensus seeming to be that they developed work independently, but the line about Prometheus is nice anyway – Leibniz’s notation is closer to what is used today. The final sentence is right on the first two suggestions, though the idea that he was “too easily satisfied with a bold and rapid glance” at any subject seems rather unfair, given the detail of most of Leibniz’s work. And of course, Gibbon was writing when a large amount of Leibniz’s work was not available; even today there is much still unpublished. But the overall assessment of his attempt at universal conquest is not far off.