"G.P." may stand for Government of Portugal. The Portuguese used Britishpattern arms in the late 1800s. IIRC - Portuguese arsenals manufactured some of their British pattern arms.
FWIW - Portuguese Snider cavalry carbines, made by B.S.A. around 1875, are marked "G.P." beneath the Portuguese crown, on the tail of the lock-plate.
IMHO - The "Nail/Rivet" repair and "Diamond shaped brass inlay" on your '1853 Enfield' pattern ML appear to be later amateur work.
Attachment 94312