I know Ullmark is currently injured, but why would Ottawa want Gibson for 2+ seasons after signing Ullmark to a new deal?
I think that Norris has negative value and nobody will take him without the Senators retaining or taking back cap+term.
Gibson is overpaid based on his play over the last few years, but he has less term than Norris and a lower cap hit.
The Senators are very clearly going to need a #2 goalie who they can trust to play 40 games a year. Ullmark has rarely played more than that and is now facing injuries.
They also need a goalie right now, and nobody reasonable is available. With Tkachuk's NMC kicking in this season, it is very important that they make the playoffs. Gibson is overpaid, but dumping Norris subsidizes that overpayment. It's not the same as straight up taking a 6.4M 1B goalie when you're dumping an 8M questionable 2C, let alone an 8M 2C who has significantly more term than that goalie.
Gibson is also a more dynamic asset. If he goes to Ottawa and plays above average hockey, he only has 2 years left after this season. If they get cold feet about having such an expensive tandem, or if Ullmark is looking solid, there is a chance they could flip that contract to a different team. The Norris contract is going to be a lot more difficult to move.
They basically need a goalie and a D right now to save the season.