Ah, I've just learnt that the notes in the Íslensk Fornrit edition
interpret this rather differently to MM & HP.

ÍF: Algengt var að menn seldu kvikfé á leigu og ávöxtuðu
svo eignir sínar. "It was common for people to lend out livestock and
get interest on their property."

Eysteinn comments: "I don't think there is any implication of naivete
here ... If the ÍF footnote is correct, Gunnar has livestock hired
out on various farms (leigustaðir) and collects "rent" from them.
This would mean that MM/HP have either misunderstood this, or that
there is disagreement as to the meaning."