In answering this question, it's important to bear in mind that Gatsby's observation comes just after Nick's remark that Daisy has an indiscreet voice. Gatsby's remark that Daisy's voice is “full of money” is a clear indication that her penchant for making indiscreet remarks is indissolubly linked to her wealth and high social status.
Because Daisy is rich and privileged, she feels unconstrained in what she says. This attitude is mirrored by her actions, which more often than not show a total disregard for other people. The most obvious example of this, of course, would be Daisy's failing to take responsibility for the death of Myrtle Wilson, even though she was behind the wheel of the car that mowed Myrtle down.
Daisy believes herself a class apart, unconstrained by what she regards as the petty rules and standards that apply to those less privileged than herself. And it is somewhat inevitable that everything about her should in some way reflect this entitled attitude. That includes the sound of her voice, which has a distinct jingle of money about it.
To Nick, this may be a source of “inexhaustible charm,” but most of us are likely to conclude that there's also something dangerous about it, something that reveals a certain contempt for basic moral norms and values.