Magical Realism
The overall book is a realistic portrait of a young man growing up in pre- and postwar Poland and Germany, filled with authentic detail about real people and places. But within this otherwise realistic backdrop, there are lots of events that seem to defy logic: Oskar's ability to not only break, but delicately carve and cut glass with his voice; suicide by fish; Oskar's decision to will himself to grow or not to grow; his being born with an adult mind; conjuring up the Polish cavalry by drumming.
Magical realism is different from fantasy (say, Lord of the Rings) because you only get something magical happening every now and then in an otherwise everyday world. Another example: the moment when Oskar stands in front of a statue of Baby Jesus:
I was ready to run like ten devils down the steps with no thanks and away from Catholicism when a pleasant but imperious voice touched my shoulder: "Dost thou love me, Oskar?" (28.49)
Weird. In other words, magical realism.
In an interview with the BBC, Günter Grass said that many of the things in the book that many readers see as surreal or odd were actually quite real, like the circus performers. When he was in the army, a musical circus of dwarf clowns performed for his unit at the front. The surreal images of the soldiers hanged from the trees was also something he witnessed. He even said that everyone knows that certain people can shatter glass by singing. He even admitted in his 2006 memoir that he once tried to get out of army drill by drinking hot oil from sardine cans and looking jaundiced. So there's a kernel of truth in many of his fantastical images, but his dreamy and poetic language makes these events and images seem almost supernatural and bizarre.