Hi and welcome to the forum.
Your item, which is, I think, a butter dish, is electroplated as you suspected. It was made by Boardman, Glossop & Co (the Gothic letters on the manufacturer's mark are "B & G"). GS probably stands for German Silver which was a term for nickel silver, a base metal alloy, and EP is for electroplate. See http://www.925-1000.com/silverplate_B2.html
. "4002" is probably a pattern number or some other form of factory identification.
We prefer to see embedded pictures in posts so for those who prefer not to click on links here are a couple of your pictures cropped and resized as necessary: