How does the author use allusion in the novel, Gravity's Rainbow?

Pynchon's novel is full of historical and cultural allusions from all over the world, and many of them are historically accurate details. Characters such as Walter Rathenau, numerous World War II episodes, corporate references to Shell, General Electric, and the German chemical company IG Farben, and stories from colonial Germany, Holland, and Russia are all based on thorough historical research. However, Pynchon often mixes these elements of historical fact with fiction, so it is difficult to tell what has been invented and what has been taken from factual historical accounts.