Reference books, i.e, Bennett's 'Collecting Irish Silver' and Bowen & O'Brien's 'A Celebration of Limerick Silver' only show show G.M in a wavy rectangle. Jackson's shows this mark too but also GM (without pellet) in a different wavy rectangle with the comment 'noted by Jackson' but I've never come across any silver with this latter mark.
This web site shows G.M in a rectangle https://www.925-1000.com/IrishProvincial_02.html
which the Jim Kemmy Municipal Museum (in Limerick) also attribute to George Moore http://museum.limerick.ie/index.php/Det ... ct_id/6520
. This mark does not appear in the Irish silver reference books mentioned above. However, the mark on your spoon seems to have cut corners to the rectangle so not quite the same.
Having said that, the fact that is stamped twice in this manner does suggest it could be Irish Provincial and probably by George Moore. Perhaps you could forward images to Limerick Museum and see what the curator says.