Take Freud with a pound of salt. He was looking for some symbolism, any symbolism, that would displace the Judeo-Christianity he hated.
Well, he found some, even if he had to hark back to Greek mythology, and suddenly everyone who wanted to be known as a psychologist was genuflecting at that altar. And mankind became stupider rather than wiser.
To him, the gun symbolized the penis. As Siggy himself noted, however, sometimes a cigar is just a cigar.