Tom is saying that the fling Gatsby had with Daisy and the whole basis of his dream is dead. He also trivializes Gatsby's love by calling it a "flirtation". Finally, the very end of the book eloquently states the inevitability of Gatsby's dream for the past. "So we beat on, boats against the current, borne back ceaselessly into the past." Gatsby was a boat against the current through most of the book as almost everything worked against him to get Daisy again. Not only was Gatsby borne back into the past, he could never escape it. His journey into the past and his dream ultimately destroyed him, and took his present and future.
Willy Loman, on the other hand, never enjoyed the success or wealth that Gatsby encountered in his life. His dream was based in an illusionary past in which he subconsciously tried to create a satisfactory past. Willy outright lies and has hallucinations and visions in his desire to have a past that is fitting according to his American values about wealth. One of his outright lies is directed to his job, where he often claims he was much more successful than the reality. "In 1928, I had a big year". Willy claims he had a big year, but the reality is given by his boss, who is laying him off. "Now, Willy, you never averaged-". This exchange is a crucial point in Willy's life. While Willy is blabbering almost about his success in the past, many years in the past, as the book is set in the 1950s, his boss is firing him because he never earned much money at all. This juxtaposition of Willy bragging about false wealth and the reality of him losing his job because he never really earned much at all is very telling of Willy's lies and attempts in-vain to create a satisfactory past for himself. Another event that shows the illusionary nature, and his failure and building a satisfactory past for himself is his funeral. Willy commits suicide in an attempt to win "diamonds" in the form of insurance money for his family.