The inspiration for writing a review on this card resulted from a mistake I recently made while submitting a deck. I published a deck that was supposed to include this Gandalf, and instead included this Gandalf by mistake! Since I had already published the deck, I decided to throw in "mistake" Gandalf just to see how it worked out. As it turns out, his performance was stellar, and helped me to remember why I've always loved this card.
The first thing that stands out is that he has built in action advantage, an ability hard to come by with allies. Not only is he questing for "free" every round with a stellar 4 willpower, but he can also block or help take down even the stoutest of enemies in the same turn. It is notable that his cost/stat value ratio is incredible. An average ally is expected to have stat line equal to double it's cost, and a high value ally typically will have 1 extra stat point (i.e. 3 cost with 7 stats). Gandalf is 6 stat points over!! The best part is that his action advantage allows you to take full advantage of this incredible value. It makes great sense thematically also because, well, it's Gandalf. He shows up, guides the way, handles a tough situation, and then leaves for the discard pile. A wizard arrives and leaves exactly when he intends to.
The threat reduction necessary to keep him in play is just enough to ensure that you don't want him in play the entire game, and yet not enough to be prohibitive. I've found that the sweet spot for Gandalf is in two opposite deck archetypes. In a secrecy archetype, or one that relies on low threat to initially fuel deck mechanics, he is great for playing just after you have crossed the threshold where you are no longer gaining advantage from low threat (i.e. threat past 20 in secrecy)... and then leaving him in play for the majority of the game, since your threat should still be relatively low. I once built a successful (and thematic) Hobbit secrecy deck around this idea.
The other great archetype that he fits in is one that is focusing on the valour trait, in which Gandalf not only functions as a very powerful ally, but can actually help you manage your threat to approach 40 and trigger valor at just the right moment. In any deck, he is a clutch play when hitting the table at just the right moment of need and then hanging around for a couple of turns.
It's cards like this that require a little bit of finesse and timing that are the most enjoyable to unlock the full potential of.