This novel is fiction. Many critics will call it historical fiction or realism. During the migration to California, Steinbeck did spend time in the migrant camps speaking to those who have traveled from places like Oklahoma to California in search of work. He learned first hand about the lives of these individuals. Even though the Dust Bowl and The Great Depression did happen, the story surrounding these historical events is fictional. Because Steinbeck took the time to actually learn and document actual characteristics of migrants, it enriches the story to make it seem real. That is the essence of a good writer.
No, to be non-fiction the novel would have to be a true story. While the story is very realistic and is based on an actual historical period and the types of events that occurred during that period, and could be, therefore, classified as realism, realisitic fiction, historical fiction, or even social commentary, it is definitely not an accounting of actual events or an actual family. So, it cannot be considered nonfiction.
You could classify the Grapes of Wrath as historical fiction. While many of the things that happened in the book were based upon real things, the book itself is a fictional work. Many people did lose their land rights in Oklahoma during the Dust Bowl. Many people did travel west trying to find a better life. Those caravans and work farms did exist, just not necessarily in the exact way as Steinbeck depicts. Steinbeck did alot of research on the Depression and the Dust Bowl. This is what makes his works on that time period so real and so wonderful. As you read his stories, you can see it and believe it.