The Collaborative International Dictionary of English v.0.48:

Gainage \Gain"age\ (?, 48), n. [OF. gaignage pasturage, crop, F.
   gaignage pasturage. See Gain, v. t.] (O. Eng. Law)
   (a) The horses, oxen, plows, wains or wagons and implements
       for carrying on tillage.
   (b) The profit made by tillage; also, the land itself.
       --Bouvier.
       [1913 Webster]

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

GAINAGE, old Eng. law. It signifies the draft oxen, horses, wain, plough,
and furniture for carrying on the work of tillage by the baser sort of @soke
men and villeins, and sometimes the land itself, or the profits raised by
cultivating it. Bract. lib. 1, c. 9.