Could Jabez Wilson be described as "gullible"? Why or why not?

Jabez Wilson might be called "gullible," but this does not seem to be the character trait that the author Sir Arthur Conan Doyle was seeking to suggest. Near the end of the story Sherlock Holmes, when discussing the case with Watson, uses a better term to describe Wilson:

“It was perfectly obvious from the first that the only possible object of this rather fantastic business of the advertisement of the League, and the copying of the Encyclopaedia, must be to get this not over-bright pawnbroker out of the way for a number of hours every day."

This is a polite way of saying that the pawnbroker is unintelligent. But if Wilson were truly "gullible," then he wouldn't have treated the Red-Headed League scheme with so much skepticism and resistance. And if he were truly "gullible" he could hardly survive in the pawnbroker business. He is actually a rather cagey, suspicious man, although he is dumb enough to be taken in by a pair of really smart operators. Wilson does not fall for their trick hook, line and sinker. His assistant practically has to drag him up the stairs to the office where the assistant's accomplice who calls himself Duncan Ross is waiting to hand Wilson the job on a silver platter.

Here are some examples of Wilson's skepticism and resistance with regard to the Red-Headed League:

“‘But,’ said I, ‘there would be millions of red-headed men who would apply.’”

When I saw how many were waiting, I would have given it up in despair; but Spaulding would not hear of it.

“‘What do you call purely nominal?'"

“Well, I thought over the matter all day, and by evening I was in low spirits again; for I had quite persuaded myself that the whole affair must be some great hoax or fraud, though what its object might be I could not imagine."

“Well, to my surprise and delight, everything was as right as possible." 

Doyle knew that one of his hardest jobs as a writer would be to sell the preposterous idea of the Red-Headed League to the reader. The best way to do this would be to make Jabez Wilson himself express disbelief. Therefore, Doyle wouldn't have wanted Wilson to seem too gullible, too easy to fool. Once Wilson has accepted the whole story about the red-headed American millionaire philanthropist founding such an association and has actually sat down and started to work on copying articles from the Encyclopedia Britannica, the reader cannot help being convinced along with him. And the final proof of the league's legitimacy comes at the end of the first week:

“This went on day after day, Mr. Holmes, and on Saturday the manager came in and planked down four golden sovereigns for my week's work." 

Money talks. The average clerk in Victorian times was earning the equivalent of one golden sovereign for a week's work and had to spend much longer hours at his desk. In those days the work week was from Monday through Saturday.

So Wilson is "not over-bright," but he has learned from experience—like a lot of people who are "not over-bright"—to be skeptical, defensive, not too trusting of his fellow man. He is described as "elderly." He must have learned something in his long lifetime. To call him "gullible" would be an oversimplification. He is more complicated than that. Doyle has created a unique comical character who is both simple-minded and shrewd. Wilson is perfectly suited for the role he has to play in the story.