Welcome to the Forum.
It's a mark I don't recall seeing before. A possibility may be the Birmingham electro-plate manufacturer, Gibson & Lord, but I cannot account for the 'I'. My only other thoughts are that it may be an early mark of the partnership of Gibson & Langman, the founders of the Goldsmiths & Silversmiths Company, should this be the case, and it is only a guess, the 'I' may stand for Ireland, Belfast being the seat of William Gibson's business interests.
I'm only offering the above as a possible route for further research.
Can you post an image of the whole item?