The dictionaries aren´t overly forthcoming about this word. Can
anyone help? At least sometimes, it seems to be a synonym
for 'kot', "[humble] cottage, small farm", see example 3 here:
and "Örnólfr hét maðr er bjó á þeim boe er heitir í Garðshorni
skammt frá Tjörn" (Sturlunga saga). Google turns up a lot of
placeneames (or a small number a lot of times...), presumably also
named after some kind of farm or dwelling. But Fritzner just has
tentatively the literal: "hjørne af en indhegning", corner of an
enclosure(?), equating it with 'garðskrókr', CV "a nook of a fence".
More examples from sagas and folktales (copy lines of URL
separetely, or just go to the main address and search
for 'garðshorn').