A thematic essay is very different from a summary or critical analysis. Rather than simply recounting what happened in the book, you need to choose a specific theme, issue or idea, that informs a significant part of the book, and then look at all the ways in which the book addresses that theme. There are three dominant themes in the book you could investigate in your essay; you should choose one of the three, and investigate it in depth.
Gogol: The book begins with the protagonist being given the name Gogol, after the Russian novelist, Nikolai Gogol, and finishes with his final acceptance and understanding of that name, after a long spiritual journey in which he rejected the name and its implications and only gradually moved towards acceptance. One thematic approach to the book with be to research the Russian writer Gogol, and see ways in which his work provides structural themes, symbols, and imagery through the book. Another would be to examine the discussions in the book of how Gogol and his family feel about his name against the significance of public and private names in Bengali culture.
Immigration and Assimilation: A major theme of the book is one of how immigrants to the United States negotiate the tensions between the two cultures of their native land and their new home. As this is a fairly broad theme, you might want to narrow it, focusing on, for example, how this tension affects romantic relationships or careers in your theme essay.
Gender: The book addresses issues of how gender expectations differ in Bengali and United States cultures. In each of the major relationships in the book, that between Gogol's parents, and the relationship of Gogol first to Maxine and then to Moushumi, there is constant tension between the notion of the roles of gender and extended family in Indian culture and the United States tradition of the nuclear family and female independence. A possible theme essay would cover how these differing cultural expectations play out in each of the three relationships.