Today's descendant's of the Gloucestershire Volunteer Artillery are 266 (Gloucestershire Volunteer Artillery) Battery Royal Artillery, still based in Bristol, at the Artillery Grounds, Whiteladies Road, Bristol BS8 2LG. The problem is that the Gloucestershire Volunteer Artillery have gone through many changes over the past century including re-badging and re-rolling, so I think that 266 Bty are highly unlikely to have many records or artifacts from 1903. Worth a try though.
I think it is equally unlikely that either the Gloucestershire Country Record Office and the Bristol Archives will have anything relating to awards of MSMs. For instance, a search on the Bristol Archives only returns 16 items for 'Volunteer Artillery', none of which look promising.
If you haven't already seen it, the Wikipedia article on the
Gloucestershire Volunteer Artillery may be helpful for finding further information.