The symbolism in Kafka is complicated. It is open and subject to almost all kinds of readerly manipulation. Gregor's transformation has been seen in many different ways over the years by critical thought. Here are some perspectives--
1. The most famous of these interpretations is the existentialist or the absurdist one whereby Gregor's transformation seems to be evocative of an arbitrary human condition in a world which is absolutely contingent and where all logical processes of signification fail, as it were.
2. One can see the metamorphosis of Gregor as a parody or may be, even a pastiche (blank parody i.e. where the staunch satirical intent of parody is missing) of the classical and epic proportions of Ovidian metamorphosis.
3. The alienation in Kafka's work and Gregor's alienation through transformation have often been seen as the representation of the alienation of the individual in a capitalist socirty. Walter Benjamin, Raymond Williams or Deleuze-Guattari's readings of Kafka in socialist and ethnic terms should be followed here.
4. The metamorphosis definitely has Darwinian connotations of animality. The failure of rational order in Kafka leads to an absolutely instinctive register as that of the beast in the story.
5. One can see the beast as a psychoanalytic fantasy because the text does mention the uneasy dreams Gregor did have on the fatal night.
6. There is a Biblical symbolism of The Fall at work. Recall the scene in particular, where an apple is placed on Gregor's head and targeted.
7. One may also apply the dehumanization theory that works in the structuralist and psychoanalytic canon of 20th century with anti-humanists like Lacan and Strauss and the idea of the disappearing human subject under the speech-act, under language, under Foucaultian discourse.
Gregor's transformation was due to his alienation and lack of validation through traditional successes (money, relationships, acquisitions). He felt small and unaccomplished, like a small bug. He felt relief in coming to terms with his status, an existential reckoning, which brought authenticity to the entire situation of his life.