There's an outside chance that the Celtics could see both players aquired from the Lakers retire before training camp. So what happens to us if they do? Larry Coon offers us insight:
Any money paid to a player is included in team salary, even if the player has retired...It is at the team's discretion (or as the result of an agreement between the team and player) whether to continue to pay the player after he has retired.
Translation: Gary won't get paid and his number comes off our cap. The link lists examples and talks about salary cap relief that can be had if an injured player retires. However, the relief doesn't come for 2 years, which makes it moot in the case of Rick Fox. In Rick's case, I believe the C's owners would pick up the insurance money on his contract, and Danny would shop it around the league as an expiring salary.
Personally, I doubt very highly that Gary simply walks away from the game. Danny has 60 days to get a feel for if Gary will report or not. If he doesn't want to play for the Celtics, and is willing to be shipped West, Danny will have had those same 60 days to have conversations with several teams looking to pick him up. That's why Danny is in no hurry for Gary to report.
It seems that Gary's threat is a good one, from his perspective, because it gives him some negotiating power. He can veto any trade by simply saying he'll stay home instead of reporting to the new club.
(p.s. - please correct me if I'm wrong in my interpretation of the salary cap rules)