Gay doesn't make sense for Boston because he isn't going to make them a contender. Now if Boston could unload some bad contracts in the deal (like Bass, Green, Terry and/or Lee) then it makes a little more sense, but it doesn't for Memphis.
A 3 team trade makes a lot more sense where Gay goes to the third team and Boston picks up some assets and/or salary relief.
These trades would make sense
Trade 1
Boston - Dudley, Johnson, Brown, PHO 1st
Phoenix - Gay
Memphis - Pierce
Trade 2
Boston - Butler, Odom, Bledsoe
Clippers - Gay
Memphis - Pierce
Trade 3
Boston - Henson, Dalembert, Dunleavy
Milwaukee - Gay
Memphis - Pierce
Or trades of a similar nature.