The Collaborative International Dictionary of English v.0.48:

Gabel \Ga"bel\, n. [F. gabelle, LL. gabella, gabulum, gablum; of
   uncertain origin. Cf.Gavel tribute.] (O. Eng. Law)
   A rent, service, tribute, custom, tax, impost, or duty; an
   excise. --Burrill.
   [1913 Webster]

         He enables St. Peter to pay his gabel by the ministry
         of a fish.                               --Jer. Taylor.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

GABEL. A tax, imposition, or duty. This word is said to have the same
signification that gabelle formerly had in France. Cunn. Dict. h. t. But
this seems to be an error for gabelle signified in that country, previously
to its revolution, a duty upon salt. Merl. Rep. h. t. Lord Coke says, that
gabel or gavel, gablum, gabellum, gabelletum, galbelletum, and gavillettum
signify a rent, duty, or service, yielded or done to the king or any other
lord. Co. Litt. 142, a.