You can only achieve a thigh gap if your bones are structured that particular way. Some people can only tell if they have a "hip dip" and if their hip bones are wide. Some people can have them, some can't. It isnt usually weight that keeps you away from a thigh gap.
You should go here: http://smellslikegirlriot.tumblr.com/post/41127647874/what-are-thigh-gaps-and-why-you-probably-arent
Some people never get a thigh gap. I didn't have when I weighed only 85 lbs... That said, lunges can help, so can lying on your back and doing the bicycle scissor kick. There are also some yoga poses that can help.
I think lunges really help but thats only my opinion :)