The thigh gap has little to do with the skinniness of the legs. It has everything to do with the position of your hips. Having a big bum doesn't affect it. I have a thigh gap (19.5" for the OP) and a big bum. The reason I have a thigh gap is because I have wide hips - 36" in comparison to my 24.5" waist.
So, for everyone saying that thigh gaps are unattractive because they mean a girl won't have a good bum or her legs will be too skinny, in all honesty, you don't know what you're talking about.