Don't ruin a good character by shoehorning him into everything.
Don't get me wrong, Gandalf should certainly be in some of this (especially hunt for Gollum), but I don't think he should be added just for the sake of adding him.
I actually really like RoP overall, but by far my least favourite part is them shoehorning Gandalf and Hobbits into it, likely because executives are like "if it's middle earth, you have to have Gandalf and Hobbits. That's what audiences expect!!"