I feel unsure that this particular GR represents a royal cypher/monogram.

Looking at cyphers etc. for George V (crowned 1911) and George VI (crowned 1937), they are far more italicized/stylized and florid than the letters shown here, or on the linked sellers carafes. Usually, they incorporate crowns and an indication of which George they are.
I've no idea how the ebay seller attributes theirs to 'government issue' and 'for military use', or why they should settle specifically on the 1930's - 1940's - their carafes are certainly simple in shape, but they don't shout art deco.
No quoted source is offered to substantiate any of these comments, although it's always possible the quoted date is correct.
Greg's reference to the 'arrow symbol' refers to the British Governments use of a very broad headed symbol used by the war department/ministry of defence over a very long period of time - pic attached, and this is sometimes seen on wooden and metal tools etc. found at boot sales.
If Greg has seen glass with initials and a separate arrow-head, then I'd agree that does sound to be war department/government issue of some description - I can't recall ever seeing glass with that broad arrow-head symbol.
I'd have thought more likely that this GR has a reference to some commercial institution/hotel/pub rather than being a Buckingham Palace throw out
