Jay Gatsby replaces his help, presumably, with members of Meyer Wolfsheim's criminal organization because they certainly were not what one would describe as "help."
Nick's maid told him that Gatsby "had dismissed every servant in his house a week ago and replaced them with a half dozen others" and that, according to a boy who had been in Gatsby's house, "the kitchen looked like a pigsty, and the general opinion in the village was that the new people weren't servants at all."
Gatsby claims that the new hires are there because Daisy was going over quite often and he "wanted somebody who wouldn't gossip."
Gatsby also claims that he hired them out of charity because "They're some people Wolfsheim wanted to do something for."
But interpretations of this scene have varied. In the most recent movie version of The Great Gatsby, Baz Luhrmann interpreted this decision to replace the help as something more nefarious. Perhaps Gatsby had put aside his duties to Wolfsheim's criminal organization in order to spend more time with Daisy, so Wolfsheim wanted some of his people to keep an eye on Jay?
Like everything in The Great Gatsby, the truth in this instance is not clear and open to interpretation.