who can explain the meaning of the verse "I wot he has garred iron gae" (in eng. "he has made cold iron go!")

from stanza
He's taen oot his lang, lang sword
That he had strappit through the strae
And through and through Clerk Saunders' body
I wot he has garred iron gae

"To make cold iron go" is an idiom?
Grazie mille