The problem isn't with Gagner per se, it's with his fit on the club.
The top 6 is full of skilled, offensive players who are either average or below average defensively outside of the possible exception of RNH. There's not a lot of physical, 2 way ability with these guys so against teams that clog up the neutral zone and take the body, the Oilers struggle immensely so something needs to change in the top 6.
Obviously, Hemsky would be the first guy that comes to mind to deal away but i believe that there's more of a market for a young player like Gagner than an injury prone player like Hemsky so with Hall, RNH, Eberle and Yakupov being ahead of Gagner on the depth chart, it's only natural to want to upgrade Gagner's position.
There is no way that a group of Hall, RNH, Eberle, Yakupov and Gagner will succeed at a high level IMO especially with the NHL turning into a grinding league. There's simply not enough all around diversity in that group.
Gagner is a good player and he should have a nice career but it will likely be elsewhere.
With that being said, they shouldn't look to deal him unless they have a contingency plan in place so he should be here for at least another year.