The Italian sounds represented by these spellings don't exist in English. Italian gn makes a sound approximately like the ny in the English word canyon, but not exactly. To come closer to the Italian sound, the tip of your tongue should touch the back of your bottom teeth.
Similarly, Italian gl makes a sound sort of like the sounds in the middle of the word "million", but not quite the same. To come closer to the correct Italian sound, the tip of the tongue should touch the back of the bottom teeth.
Z is sometimes voiceless, like ts is bets, cats. Example:
Z is sometimes voiced, like ds in beds. Examples:
(*) In either case, its sound differs more distinctly from s than it does in English.
S is sometimes (but always before b, d, g, l, m, n, r, and v) like the english s in easy, or the the English "z" in zoo. Examples:
S is sometimes strong and hissing like the English "s" in house, set, strip. Example: