Hello Inquisitive Collector, I think most collectors with an interest in silver plate would put your marks in with the "J.Gilberts" of Birmingham marks. The J.G mark being somewhat difficult to sort between John Gilbert and Joseph Gilbert. The "Gilbert" name is associated with a very large number of different pseudo hallmarks though "Gilbert" is marked on a few pieces with the other marks and used to confirm an association with "Gilbert" where the name is not present. I have loaded a photo to show association.
The top set of marks is very like the marks on your spoon but has the mark of a "cross on top of a triangle" mark substituted. The lower set of marks most people would believe is a mark of "J.Gilbert" of Birmingham and our reference section says "John Gilbert". I am not as confident as our reference section or "legrandmogol" as to which "Gilbert". Chronologically speaking a "John" item would be older than a Joseph item.