When Nick says this to Gatsby, or rather when he shouts it at him across the lawn, it is after Gatsby has taken the blame for Daisy hitting and killing Myrtle Wilson in his car. After this point in the story it becomes obvious that Daisy will stay with Tom and abandon Gatsby. Daisy retreats to the security and protection that Tom can offer her, leaving Gatsby to take the blame for Myrtle's death.
When Nick shouts to Gatsby that he is "worth the whole damn bunch together," he means that Gatsby is better than Tom and Daisy and everyone else who lives, entitled, cynical, superficial lives, as Tom and Daisy do. This includes the people who flocked to Gatsby's lavish parties to take advantage of his generous hospitality and who don't bother to attend his funeral later in the story.
At the beginning of the story, Nick, narrating retrospectively, says of Gatsby that "there was something gorgeous about him, some heightened sensitivity to the promises of life." Nick also says that Gatsby had "an extraordinary gift for hope," and "a romantic readiness" which he "never found in any other person." We might infer from these quotations that Nick thought Gatsby to be hopeful where others were cynical and romantic where others were crudely pragmatic. In the same passage of the book, Nick says that he became disillusioned with and scornful of "what preyed on Gatsby, what foul dust floated in the wake of his dreams." The implication here is that Nick thought Gatsby to be "worth the whole damn bunch together" because while they preyed on him, he offered them only hope, romance, and beauty.
This seems like a fair assessment, for the most part. Gatsby doesn't "prey" on anybody else in the book. He always acts in pursuit of love—or at least what he thinks is love—for Daisy. He accrues wealth as a means of obtaining Daisy, and other people benefit from that wealth.
The other characters, however, and most notably Daisy, do indeed "prey" on Gatsby. Daisy toys with his love for her and then lets him take the blame for Myrtle's death. She sacrifices him for her own sake, and Tom is complicit in this sacrifice. In a broader sense, these characters (Tom, Daisy, and all of the people who attend Gatsby's parties) represent the superficial materialism of the American Dream, and it is this which truly preys on Gatsby. The characters are merely conduits, and perhaps even victims themselves, of the corrupting, debasing ideology of the American Dream.