one will need to make a comparison between the date the maker was operational and the date year stamped on the ring. going on the written information provided, it could be one of a few British maker's with initials J.G.
There should be another letter in the hallmark series, at the end, which is the date letter, this will provide evidence to either confirm or refute the maker - John Gilbert & Co. (General Silversmiths, Birmingham, UK) - ca 1850s - 1880s
