Plato famously maintained that knowledge is “justified true belief,” meaning that to claim the status of knowledge our beliefs (say, that the earth goes around the sun, rather than the other way around) have to be both true (to the extent this can actually be ascertained) and justified (i.e., we ought to be able to explain to others why we hold such beliefs, otherwise we are simply repeating the — possibly true — beliefs of someone else).
Plato did not, in fact, maintain any such thing. In the Theaetetus, in which he discusses various possible accounts of knowledge, Plato's Socrates considers three possibilities: sensation, true judgment, true judgment with logos. Socrates rejects all three -- each one has problems. The major problem with the last (which is introduced not by Socrates but by Theaetetus as something he has heard elsewhere) is that no plausible account of a 'logos' actually gets us anywhere -- every way 'logos' is cashed out (Socrates tries out several different ways) 'true judgment with logos' ends up being either circular or obviously incomplete. That is, it always just ends up meaning 'true judgment that is knowledge' or it leaves something out, making knowledge indistinguishable from certain kinds of lucky opinion. Socrates concludes that the account is silly and a bunch of wind, and there the dialgue ends as Socrates heads out to his trial. The most plausible account of knowledge attributed to Plato himself, of course, is that knowledge is rational perception of the unchanging Forms.
JTB theories of knowledge are in fact quite minor accounts historically; Gettier's arguments against them have no purchase against Neoplatonist or Aristotelian accounts of knowledge, nor do they work against either empiricist or rationalist accounts of knowledge in the early modern period, and that covers the major accounts of knowledge in the history of Western philosophy. Like many of the things contemporary philosophers take as obvious, JTB does not have a very extensive history, and there is no clear point at which people picked it up because it was superior to any rivals. Indeed, as seen from the rather extensive misattributions of it to Plato, it often seems to be go with a complete ignorance of the fact that there are any rivals. But the history of philosophy consists almost entirely of its rivals. Most positions take knowledge to be some kind of perception or apprehension; Aristotelian accounts take it to be logical derivation from immediate principles, which themselves are directly apprehended. There's a good argument to be made that almost everyone prior to at least the nineteenth century would have taken "justified true belief" to be a nonstarter as an account of knowledge for reasons broadly similar to those found in the Theaetetus.
