Gandalf's powers

[Eönwë]

Gandalf does have powers and he can make fire. He is a Maia of the air, so I don't think he could change Sam into a toad. Also Frodo is laughing in this scene, so we know that Gandalf can't really, or at least would never, even if he had the power.

But I do wonder... how did he interrogate Gollum?

[Legate of Amon Lanc]

Originally Posted by Eönwë

He is a Maia of the air

Huh? What's that supposed to mean?

There is no such classification as far as I am aware, and even if it were (somewhat through the Valar), Gandalf is said to belong to Nienna and Lórien (am I right?). I don't know about them having anything to do with air, however.

But I do wonder... how did he interrogate Gollum?

First by kind words, then by hard words and at last intimidating him by performing fireworks.

Originally Posted by Shadows of the Past

I endured him as long as I could, but the truth was desperately important, and in the end I had to be harsh. I put the fear of fire on him, and wrung the true story out of him, bit by bit, together with much snivelling and snarling.

I imagine it very similar to what Gandalf did to Bilbo Baggins when he persuaded him to give up the Ring. Just instead of Gandalf growing up to the ceiling imagine flames rising around him. Otherwise the scene could have looked pretty similar, I believe.