A brief explanation (and I'll try to keep it simple): Teams that offer their free agents salary arbitration get Draft picks in return if the player signs elsewhere, and the number of picks is based on the player's rating. To get compensation, a team must offer arbitration to its Type A or B free agent and have that player either reject arbitration or sign before the deadline to reject the said arbitration.
The Elias Sports Bureau determines the designations based on statistical study. Gregg is a Type A free agent, Harden is Type B. Johnson and Fox are non-compensatory. So, if the Cubs did offer arbitration to Gregg and another team signed him, the Cubs could get two Draft picks. One would be a first-round pick from the signing team (if the team's pick is below the first 15 picks in the Draft) and a supplemental pick. The supplemental pick is a player taken between the first and second rounds.
If the signing team has a pick in the first 15 slots, the team that loses the free agent will get the signing team's second-round selection and the sandwich pick.A Type B free agent like Harden would be worth one Draft pick to the Cubs if he signs with another team. That pick is a supplemental pick.
I'm not quite sure how Gregg rates Type A and Harden Type B, but there it is. The Cubs clearly don't want Gregg back, so they won't offer arb even though two picks would come in return, because he'd likely accept and the Cubs would be stuck with him. Chad Fox, obviously, will likely retire. Since Reed Johnson is non-compensatory, it makes no sense to offer him arb -- if the Cubs want him back, they can negotiate with him as a complete free agent.
The official deadline is tomorrow; we'll know more by then.