Brewer's: Gregorian Tree
The gallows; so named from three successive hangmen—Gregory, sen., Gregory, jun., and Gregory Brandon. Sir William Segar, Garter Knight of Arms, granted a coat of arms to Gregory Brandon.
This trembles under the black rod, and he Doth fear his fate from the Gregorian tree.
Mercutius Pragmaticus (1641).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894