As a 'gloggy', my reference in this matter is always the - for ever on sale- booklet "Scottish Place Names" by George Mackay.
This is what is says:

Glassery (Argyll and Bute) 'Grey Shieling'. Glas (Scottish Gealic) 'grey'; airidh (Scottish Gealic) 'shieling'.

It's in line with BigSweetie's -not so rough- guess!

Cheers,

Jan