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?

Hi Dimitry, I misread your question. Valerie has responded well. Thanks, Stephen