I've been out of school long enough that the more challenging aspects of English grammar are difficult for me to remember.
However, I will give this a shot. Let's say the same thing, but turn it around a little bit so you can more easily see what I am trying to explain. :-)
"It's a great treat, listening to his travel stories."
"Listening to his travel stories is a great treat."
Let's remove the prepositional phrase, 'to his travel stories' which will make the basic sentence more easy to see.
"Listening is a great treat."
Let's remove the adjective that describes the treat, as well.
"Listening is a treat."
Now that we are looking at it in its simplified form, we can see that the subject of the sentence is about a thing (noun) that is called 'listening'.
Does that make sense?