Answer :
Cambridge English Dictionary defines "gasp" as "to take a short, quick breath through the mouth, especially because of surprise, pain, or shock."
The word "gasp" affects the tone of the given excerpt in the following way :
It conveys a tone of surprise.
The short story “About Russell” by Rita Williams-Garcia is about a boy suffering from mental illness. He has been exhibiting abnormal behavior like not making many friends, and other types of behavior that doesn't make sense to other people. He is a mathematical genius and an introvert who doesn't like mixing with people.
The speaker and Rosalind are surprised when they find out that a girl had come over to meet their brother Russell and he was having an actual conversation with her. They are completely surprised with this development.