Gucci Mane hit the main stage just after 6 p.m. Saturday (Aug. 19) at the Billboard Hot 100 Festival riding off a DJ set that delivered cuts from Migos, 21 Savage, Lil Uzi Vert and, most significantly, "Classical (Intro)," off his 2009 major-label debut, The State Vs. Radric Davis. And when he arrived he came out firing, starting out --…