I am not too sure that gullibility is the right word to use in reference to this story. After all, there is no sense in which the narrator believes the story he is told. In fact, it is clear that he sees he has been told a "tall tale" and does everything he can to escape being barricaded in to hear yet another interminable tale. Instead, it might be better to use the word deception. After all, deception seems to operate on many different levels in this excellent story. Note how the narrator only goes to speak to Simon Wheeler at the request of a friend, who, we begin to suspect, did it primarily to annoy the narrator. Likewise the narrator is at first deceived by the serious tone of Simon Wheeler that makes him think he is going to be told a sincere narrative:
He never smiled, he never frowned, he never changed his voice from the gentle-flowing key to which he turned the intial sentence, henever betrayed the slightest suspicion or enthusiasm; but all through the interminable narrative there ran a vein of impressive earnestness and sincerity, which showed me plainly that so far from his imagining that there was anything ridiculous or funny about his story, he regarded it as a really important matter and admired its two heroes as men of transcendent genius in finesse.
The narrator is definitely taken in by this at first, though, as the true humorous nature of the story becomes clear, it is obvious that this does not last long. Lastly, of course, there is the way in which the Smiley of the internal story is deceived by his oponent who fills up his frog with lead. Thus it might be wiser to focus on deception in this tale rather than gullibility. The narrator is taken in, but only initially, and it is obvious at the end he realises the trick that has been played upon him.