Justified is the most contentious word in the JTB account of knowledge, usually it can be understood as if one has strong evidence about a proposition then one is justified to believe in it. Its wikipedea reference also says the same criterion as referenced here:
Suppose that Smith and Jones have applied for a certain job. And suppose that Smith has strong evidence for the following conjunctive proposition: (d) Jones is the man who will get the job, and Jones has ten coins in his pocket. Smith's evidence for (d) might be that the president of the company assured him that Jones would, in the end, be selected and that he, Smith, had counted the coins in Jones's pocket ten minutes ago. Proposition (d) entails: (e) The man who will get the job has ten coins in his pocket.
So in the Gettier's case above the evidence that the president of the company assured him that Jones would get the job is commonly regarded as strong evidence. Of course in reality for most real-life or scientific propositions we need much more such evidences so such edge case is very unlikely to occur. Indeed, one criticism of Gettier case voiced such insufficient evidence in the same reference like you:
Affirmations of the JTB account: This response affirms the JTB account of knowledge, but rejects Gettier cases. Typically, the proponent of this response rejects Gettier cases because, they say, Gettier cases involve insufficient levels of justification. Knowledge actually requires higher levels of justification than Gettier cases involve.
Of course there're several other approaches in the same reference to solve Gettier problem and acknowledge JTB issue to perfectly define knowledge.