If the Yankees wanted to take on a salary dump SP trade, I'm sure they could get a Lohse/Buerhle/Edwin Jackson type return.
They're not going to get a good young starting pitcher like a Wade Miley or Pat Corbin, and a team that could offer them a REALLY good pitcher with a huge contract, like Cliff Lee or something like that, would not make the trade for one year of Gardner.
Same thing if you talk about 2nd basemen, another area of need for the Yankees. The Angels would never give up Howie Kendrick - why tread water in terms of value for the next year while getting less years of team control? In a different way, the Cardinals would never give up Kolten Wong - why give up so many years of team control just for one year of Gardner?
So maybe they're better off keeping him.