Green’s Dictionary of Slang

one who spreads malicious rumours.

1709		R. Steele Tatler No. 88 n.p.: Those who recount strange Accidents and Circumstances which have no Manner of Foundation in Truth, when they design to do Mischief are comprehended under the Appellation of Gunners [...] Gunners therefore are the Pest of Society; but the Gunsters often the Diversion. The Gunner is destructive and hated; the Gunster innocent, and laugh’d at.
1727		Boyer Fr. Dict. Gunster (a Cracker, or bouncing Fellow), un Bavard.