The reference to Belasco in "The Great Gatsby" refers to real-life theatrical producer David Belasco. The mention of Belasco is made by a character called the owl-eyed man who looks at the books in Jay Gatsby's library and is surprised to discover that they are real and not theatrical props.
Taking a book down from the shelf, the owl-eyed man emphatically commends Gatsby for the artful staging of his home and equates him with Belasco. By comparing Gatsby to a theatrical producer, the owl-eyed man implies that Gatsby is only a pretender in his life and that his mansion and everything in it, including the books, is no more real than a set for a stage play. However, he admires how much detail Gatsby put into the quality of illusion. Comparing him to Belasco is a compliment on how realistically Gatsby produces the show of his life. The owl-eyed man sees that Gatsby has not opened any of his books and notes that this would be taking the pretense too far.
Belasco was a playwright, director and producer active on Broadway from the late 1800s through the 1920s. He was known for naturalism and realistic stage sets. The Belasco Theatre in New York City is named for David Belasco.