Start with the most basic similarities: both are examples of Jewish fabulism. That is to say, both are written by Jewish authors, about Jewish characters and using language marked by traditional Jewish references. The plots likewise are defined by a shared Jewish heritage. Both characters consult figures only or largely found in the Jewish community, like the rabbi or the marriage broker. They are also examples of fabulism: they are a Jewish tradition parallel to magical realism. Both stories are set in a largely realistic world, but then defined by an intrusion of the supernatural or impossible.
Next, move to the more subtle or substantial similarities. In both cases, when the supernatural intrudes, it changes the characters' lives forever. Leo only wants the girl in the picture. Gimpel is visited by the Devil, and not only doesn't give the town the tainted bread, he leaves it. In both stories, there is more than a little humor, and often the humor is self-deprecating. In both cases, there's a kind of tongue in cheek quality: we're supposed to feel for the characters, but not necessarily believe in them.
The main differences are that Finkle is not so much a figure of fun as Gimpel. The settings are different: 20th century New York vs. an older if unspecified Old World setting. And the point of view: third in "Magic Barrel" vs. first in "Gimpel."