Hmm... I think the net that the Ducks could cast is far wider than the 3-4 mentioned in the OP.
I think it might be interesting if the Ducks retained and traded him for Raanta from the Canes. Andersen and Gibson reunited.
Teams that might make sense for a contention retool/push over the top: Boston, Caps, Wild, Preds, Canes and Avs.
Boston is a team that might need to make a few additional moves to retool other parts of the roster for Gibson to make sense though.
I'd be uncertain of these teams because of NTC and inter-divisional reasons: Toronto, Edmonton and VGK. That being said, I think these 3 would be the most likely to pay a significant premium over other teams to try and acquire Gibson.
Gibson makes 6.4 for 6 more years or something. At 50% retained, that's decent AAV even for a back up and Gibson is superior to back up calibre. Retaining the contract to 3.2-4 mil AAV per year for 6 years would suck, but the Ducks could potentially ask for some pretty insane pieces that could actually make it worthwhile to retain for that long.