G.S. Wyllie was a hand-built effects pedal and instrument operation run by Glenn S. Wyllie (1948–2014), an artist and builder based in North Carolina. Wyllie traced his interest in effects to the early 1960s, when hearing the Gibson Maestro Fuzz-Tone on the Rolling Stones' "Satisfaction" set him to replicating early fuzz circuits with parts on hand. Working as a one-person shop, he handled circuit design, wiring, and enclosure finishing himself, and built pedals largely to order rather than carrying regular stock.

The shop is best known for its line of hand-cast metal-shelled fuzzes and octave effects, including the Moonrock (a one-knob fuzz with swell and octave behavior), the Fuzzmite, the OZO octaver, the Wylo fuzz, the Over-Tron overdrive-into-fuzz, and the Rising Sun, a take on the Shin-ei Super-Fuzz. The distinctive lumpy enclosures originated from an early plaster-of-Paris mold experiment with molten pewter; later runs were cast at a foundry from match plates while Wyllie continued to mill, drill, and finish each shell by hand. Alongside pedals, he also built guitars, sitar guitars, and mandolins.

Wyllie's output is collected in small numbers and trades on the boutique secondary market, with discussion threads and builder profiles on The Gear Page, Effects Database, and Reverb tracking individual units. The Moonrock circuit was later reinterpreted by JHS Pedals as the Coyote octave fuzz.

G.S. Wyllie has 1 product cataloged on Equipboard, including Fuzz Effects Pedals.