I wouldn't call it a gap. The Bullitt pillars are shaped differently. The reason for the primer area on the GT's with c pillar swaps is a result of the painting step during assembly. It appears that ford paints the c pillars and quarter panels after they are installed. The stock GT pillars stick out farther on the quarter panel and the Bullitt's are more tapered towards the glass.
I will get some close up pics of what I'm referring to. Give me a couple days with the Holidays and I will have some pics up