I actually think it's an unrealistic and unhealthy aims for most girls. Fair enough it depends where you hold your weight, but I've been rather upset lately to see photos of girls with a thigh gap being praised, when the girl in question has clearly managed to achieve an unhealthily low weight in order to get the gap.
That being said, I understand that some girls are a different build, and could be a healthy weight and still have a thigh gap, but not for most.
Promote a healthy weight, a healthy body and a healthy mind. Don't fixate on little things like that.