Yes, Ranke and Gibbon are names. Gibbon, for example, was an 18th Century British historian who wrote a famous history, “The Decline and Fall of the Roman Empire.” Putting an “a” or “an” before a name is used in this sense: “a Gibbon” means “a person with the same traits as Gibbon.” The meaning in the sentence above is that there was so much information/knowledge accumulated by a certain point in history that even the most intelligent persons of prior year (like Gibbon) would not be able to learn/understand it all. Another example: “The military situation is so bad that even a Patton [famous army general of WW2] could not win.”

I think the a in front of Ranke and Gibbon should be read as emphasizing certain properties that Ranke or Gibbon represent. I don't actually know that person Ranke, but according to this text he was a very industrious person. So he is used as an example of people who are very industrious, just as industrious as he he was, or just as industrious as a Ranke. Ranke is used as a representation of industrious people. Gibbon on the other hand must have had a very sharp mind. And he is used as a representation of that. There may be others like him. I should point out that I don't know which Gibbon the text is refering to. I assume it's not the monkey.

This is an unusual usage. We can use the name of a famous person to represent a whole category of people, and then put "a" or "an" in front of it to mean "someone else in the same category." For example, Einstein was a genius. Instead of staying "only a genius could solve that puzzle," we can say "only an Einstein could solve that problem." We are using the name of one particular genius to mean "genius." We need to use the word "a" or "an" to mean "one person in the category of 'geniuses,'" so we also need to use it for "one person in the category of 'Einsteins.'"--"an Einstein." The sentence you've given us is old-fashioned in style. I have just done a search and found out that your sentence is from "Eminent Victorians," by Lytton Strachey, written in 1918. Gibbon and Ranke were more famous then than they are now. I knew Gibbon was a famous historian. I had to look up Ranke, and found that he was another famous historian. Strachey thinks that Ranke was very industrious (hard-working) and Gibbon very perspicacious (full of insight). The sentence means there is so much material about the Victorian era, that writing a history of it would be impossibly difficult even for an historian who was as industrious as Ranke and as perspicacious as Gibbon. The phrase _could_ also be written as "so vast a quantity of information that the industry of Ranke would be submerged by it..." His choice of words is a matter of style. He probably says it that way because he's talking about himself and what he hopes to accomplish. He could say "Ranke couldn't do the job of writing a complete history of the Victorian era." But Ranke himself was dead and wasn't going to try! Strachey is of course not Ranke, but he might possibley be "a Ranke," or at least have "the industry of a Ranke. Strachey says, though, that the job is too big even for "the industry of a Ranke." So, instead of a history, he says, he will just write four short biographies.