The first office was established in Tangier in 1857; mail was simply bagged there and forwarded to Gibraltar just across the water, where it received the standard "A26" postmark.
Several examples of loose GB Queen Victoria stamps cancelled Tangier do exist including at least one horizontal strip of 6 1d reds from plate 123.This item was lot 417 in the H R Harmer sale on 12–14 December 1960.
The overprint was basically the same as for the British-currency stamps, with the added complication of needing to fit in the denomination in Centimos and Pesetas.
All types of British stamps were overprinted, the last being the issues of Elizabeth II in the summer of 1956; all were withdrawn from sale 31 December of that year.
Subsequent to that the late Dr Ken Clough produced two booklets (the second being a revised version of the first) published by the Gibraltar Study Circle in 1978 and 1984.
The Gibraltar Period up to the end of 1906 has been covered in great detail in a book written by Richard Garcia now published by the PHS/BPT.
Richard's book, together with that by David Stotter, provides complete coverage of the postal history of the British Post Office in Morocco.