Like rockart sez, its from the Gaelic 'glas airidh', being 'grey or green shieling or hill pasture.' (according to Johnston's "Place-Names of Scotland" & echoed in Cameron Gillies' "Place-Names of Argyll"); although a 1394 entry describes it as 'Glaster,' which'd be grey or green land, without inference to pasture. There y' have it!