I might swap out the word "content" for "value". I think value does a good job at unpacking the usefulness of the content of the game.
"Content" and "value" are closely related, but they aren't the same thing. Content is what determines value when we are talking about entertainment. An interesting 10-page novel for $20 is a bad value. A boring 1000-page novel for $20 is also a bad value. An interesting 1000-page novel for $20 is a great value. But the value is not the content. The content is what you actually experience when you read the book. The content is the reason why you buy.
For a book, the content is closely related to the story. But games are much more than just a story. The best word for games is just content.
Content and challenge is under gameplay
Content is different from gameplay. Mario Kart, 2D Mario, 3D Mario, Mario RPG, and Mario + Rabbids all have very different gameplay. The content of these games is very similar. In any of these games I can experience Mario, Bowser, Goombas, Peach's Castle and all of the other parts of the Mushroom Kingdom. The way I experience it is the gameplay. But the actual world and the stuff in it are the content.
Graphics are experienced with the eyes. Music is experienced with the ears. Gameplay is experienced with the hands. Content is experienced with the mind.