That would be pretty awful if Wyc blocked a trade that would benefit the team.
Fortunately, Pierce for Gay straight-up would NOT benefit the team. Even at this age, Pierce is a better player than Gay, and while Gay is younger, he is 26. By this age, it is unlikely that he is going to make a huge leap. He is what he is at this point in his career. And what he is is a worse player than Paul Pierce.
Compare their numbers:
http://www.thenbageek.com/players/compare?utf8=✓&player_ids%5B%5D=15&player_ids%5B%5D=150Pierce scores more points, grabs more rebounds, is a better passer, has better shooting percentages and points per shot, gets more steals, and draws more fouls. The only categories in which Gay has an edge are blocks (but neither are good shot blockers, and the margin of difference is minimal), turnovers (but Pierce has a higher usage rate, and again, the margin of difference is minimal), and fouls (Pierce plays more physical defense, which results in more fouls).
Why would we make this trade? From a basketball stand-point it doesn't make any sense because Pierce is clearly superior, from a money stand-point it doesn't make any sense because Pierce is on a better contract, and from an "intangibles" stand-point it doesn't make any sense for reasons I am sure I don't have to point out. And this isn't even getting into Gay's injury issues.
I very much hope this trade does not happen, and Wyc would be justified in blocking it from every possible angle.