Green’s Dictionary of Slang

1. rousing, inspiring, successful, impressive.

1996		K.F. Staley Art of Short Selling 102: The company explained the increase as seasonal buildup before a gangbuster summer.
2005		S. Forbes Flat Tax Revolution 98: In 2003, Lithuania’s gangbuster growth rate was an eye-popping 9 percent.

2. over-zealous.

1994

Rebennack & Rummel Under A Hoodoo Moon 2: Shut down by a gangbusters DA who padlocked the gambling dens, whorehouses, juke joints [...] that had kept the New Orleans music scene alive.