The purpose of all topic sentence, depending on it's location, is to:

The purpose of all topic sentences is to announce what the paragraph will be about.  Besides all topic sentences have to be related to the thesis statement in order to achieve unity and focus in a text. A topic sentence generally appears in the first or second sentence in each body paragraph and serves to unify the content of the paragraph. Paragraph unity should be  accomplished if all the paragraph sentences are related to the topic sentence. When the paragraph contains restated ideas from the whole text, the topic sentence is placed at the end as a concluding statement.

gatsby's clothes have a symbolic meaning, as the use of colour and fabric in his clothing demonstrates his hope, dreams, and wealth. gatsby's clothing symbolizes his wealth through his torn shirt, his silver and gold suit, and his english shirts.