What these words reflect about the feelings of Aunt Agatha for Gussie is that They show her disdain for Gussie's lack of self-control.
Explanation:
Aunt Agatha is a strong character who wants to make his nephews to marry women that according to her beliefs are worthy and won't dishonor the name of the family, that's the reason why she shows her disdain toward Gussie's feelings since the woman he has fallen in love with is not what Aunt Agatha would expect.
When you say that someone has lost his head over something it means they have an obsession towards that thing. Liking something too much is a form of obsession. So the feelings projected by Aunt Agatha is a concern that Gussie likes America too much.
