Solid gold pieces typically have only a number or a number followed by a karat mark, while gold-plated and gold-filled pieces have additional marks, such as "GP" or "GF." Not all gold pieces have identification marks.
"K," "KT" and "KP" all mean karat, so a mark of "18K," "18KT" and "18KP" all indicate an 18-karat solid gold piece. European gold markings indicate a piece's gold percentage. For example, a "755" mark indicates a gold percentage of 75.5. Multiplying the number by 0.024 converts it to karats.
"GP" means gold-plated, and "GF" means gold-filled. These pieces may also include a karat number before that mark, but that's only the karat of the plating or filling, not the whole piece. Jewelry stamping didn't become standard until the early 1900s, so solid gold pieces from before this time may not have identification marks.