Many manufacturers copied the famous blue onion/danube patterns and still do - I've not known GMT & Bros to be part of the Meissen marks - It appears to be British or American worded like that -
Searched the British Staffordshire potters - no GMT - google & other search engines keep giving me GMT links meaning Greenwich Mean Time. Is there any other identifying mark?
Suggest you search e-bay for cannister sets blue onion and perhaps here for the same:
www.kaleden.com in the porcelain pottery category. You may get a feel for the value of your set that way. Best wishes, KB
