1) The ‘gad’ was originally an iron spike or bar and both meanings are on record from the early thirteenth century (OED).
1311 Pro … vj gaddis asceri [6 steel bars] cum fabricacione eorumdem, Bolton Priory
1510 of ylk a hundreth gadde of steylle, York
1581 For two longe yron gaddes for staying of a pynnacle of the churche, 11s, York. In 1693, Thomas Kay had some old gad Steele and in 1701 part of a delivery of steel to the Fell partnership of Sheffield ironmasters ‘was made into … gadd steele for the cutlers’. There are some documents in which the bar seems to have been used as a standard measure, and Colin Chapman says under ‘Iron Weights’ that 30 gads were equivalent to 1 sheaf. It could also be equivalent to ‘wand’, used as a linear measure: 1609 one rod or gadd, Malham
1658 a parcel of meadow within Great Cleave 12 gadds in breadth, Skerne. Lands called le Gadds are referred to in a parcel of deeds for the period 1600-58, and the confirmation of the grant which is in Latin describes them as ‘twenty one perches of meadow’, West Butterwick.