If you want to know what is holding up the Bynum for Gasol trade, it has nothing to do with those two players, but the Lakers wanting additional assets.
According to ESPN the Lakers want a young player or 1st round pick to make the swap and the Cavs are reluctant to do that, so trade talks have stalled.
The Cavs have until January 7th to trade Bynum because at that point his contract becomes fully guaranteed and no one will want him. If they trade him before the 7th the Lakers or another team can cut him and get some salary cap relief.
If there are no takers by the 7th the Cavs will cut their loses and just release Bynum.