In Gettier's "Is Justified Belief Knowledge?", how does he provide a counterexample to oppose Plato's definition of knowledge? What is his thought experiment all about?

Edmund Gettier famously argued against Plato's definition of justified true belief being equivalent to knowledge. To do so, he used a somewhat paradoxical story. He proposes several examples.
One of his counterexamples is that he is watching the Wimbledon men's final where John McEnroe defeats Jimmy Connors. He rightly believes, after watching it, that McEnroe is the men's champion that year (because he was). However, he was actually watching the previous year's match, which had ended much the same way. So, for that example, he had true belief but did not actually have knowledge of the true events.
Another example is that two men, Smith and Jones have both applied for the same job. Smith knows, somehow, that Jones has 10 coins in his pocket, and has good evidence that Jones will get the job. Therefore, he assumes (believes) that the man who has 10 coins in his pocket will get the job. However, unbeknownst to him, somehow, he also has 10 coins in his own pocket. Therefore, his knowledge of the first facts are true (Jones got the job, and Jones has 10 coins in his pocket), but his conclusion is false.
In the final counterexample, he gives a scene where Smith believes, from the word of another, that Jones owns a Ford. He justifiably concludes from this that Brown (a third man) is in Barcelona. However, Jones does not own a Ford, but Brown is in fact in Barcelona. This states that Smith has belief which is accurate, but he does not have knowledge.
In all cases, it can be noted (and has been noted in other scholars' responses to Gettier) that the "justified" nature of true belief in all of his examples can be questioned and thus invalidates his argument. However, the nature of what constitutes a "justified" true belief is still very much in debate.