It's a grey and dark blue pinstriped two-piece suit with a matching fedora and a black-striped tie.
The outfit is only available to the members of the RockstarSocial Club. After connecting the PSN\Xbox LIVE account with the Social Club account there should be a screen letting you know the outfit has been unlocked and is available under the Outfits section. The outfit will then be unlocked and will be able to be used at any time.
Starting a New Game may require that the player connect their PSN/Xbox LIVE account again to obtain the outfit on the new playthrough.