A good example of a gap to write about is what Gandalf was doing when he disappeared.
A gap in the story could be anything. You need to think about questions you had as you were reading, or areas that are not well explained. The best example of that for this book is Gandalf. We know that Gandalf disappears at several times during the story, but we do not know why. He does not always tell us.
Sometimes we find out later what Gandalf was doing, but he is often cryptic. For example, when he helps rescue the adventurers from the trolls he does not tell them where he is going at first. He just leaves. Then he returns, and Thorin asks him where he has been.
"Where did you go to, if I may ask?" said Thorin to Gandalf as they rode along.
"To look ahead," said he.
"And what brought you back in the nick of time?"
"Looking behind," said he.
"Exactly!" said Thorin; "but could you be more plain?" (ch 2)
We learn that Gandalf goes to explore and communicate with others at several places. He always seems to know when to come back. One of the most significant times he disappears is when Bilbo is looking through the treasure and gives away the Arkenstone to the Elven king. Gandalf appears just in time, before Thorin can punish Bilbo. Where did he go?
Gandalf seems to want to give Bilbo a chance to grow and become a hero. You can see that when Gandalf goes, Bilbo often steps in. He could not really grow to his full potential if Gandalf did everything for him.
You can ask yourself, based on evidence of where Gandalf has been, how he knows when to disappear and when to reappear just when he is needed. Do you think he is nearby? Does he use magic to spy on them? How does he decide when to return? You get to decide!