In a very broad sense, a prominent theme of "Gimpel the Fool" is trying to reconcile one's beliefs with the truth of world around them (beliefs vs. reality).
On the surface, "Gimpel the Fool" tells readers a tale of what seems to be a very foolish man, one who is disrespected by his wife, his community, and employees. Yet through all of Gimpel's misfortunes, he retains belief in God and in the truth that could potentially be on other's lips ("In the first place, everything is possible, as it is written in the Wisdom of the Fathers," Gimpel the Fool, p. 1). Gimpel's beliefs frequently cause him to be made a fool of in the reality of his social sphere, although Gimpel's ending epiphany informs readers that his beliefs triumph: "No doubt the world is the imaginary world, but it is only once removed from the true world" (Gimpel the Fool, p. 10).
Thinking more symbolically, the theme of beliefs vs. reality is realized when you read the short story as a symbol of the Jewish experience post-WWII. Gimpel and the people of Frampol are Jews themselves, struggling with what seems real in the natural world and what is true in the spiritual realm. When this story was written in 1953, many Jews had been displaced and affected by the horrors of the Holocaust. Many could find pieces of themselves in Gimpel's story; they, like Gimpel, were asked to hold on to the "truth" that they were still the chosen people of God despite the trauma they as a whole had gone through.