Abby is a child-like spirit, yes, but a wounded one. She’s very guarded. Scared to get close? Affection from Gibbs is safe. Affection from McGee might lead to something more, so she blocks it. Does that make sense?
I think her job is her life & the team is her family. She doesn’t want to risk that even though she obviously cares for McGee. Maybe one day she’ll get brave! Or McGee gets tired waiting and calls her on it. Both would be bold moves from their characters.
I have no doubt she has a little place in her heart for McGee she doesn’t have for the others. I just think they have to evolve a little more for that ship to happen… don’t think it’s impossible. Time!