Answer:
Garth's refusal is based on his gender schema.
Step-by-step explanation:
Gender schema refers to the general gender-related beliefs which in turn influence how an individual behaves. It is the way an individual thinks about genders and how that influences his/her behavior.
In this case, Garth's belief is that girls don't and can't play with trucks because it's a boy thing. His gender-related belief is influencing his behavior and is why he tells Amelia she can't because she is a girl.