GH
GH, gh is a digraph (a two letters grapheme) used in various languages using the Latin alphabet, with different values, especially in English, Irish Gaelic, Scottish Gaelic, Cornish, Italian, Romanian, Friulian and Corsican.
Use in English
GH is a notorious digraph in English in such words as nîght and cóugh (pronounced *nîte and *cóff: the accents show pronunciation: see English phonemes). It is the sorry relic of a sound (IPA χ) no longer pronounced except in exclamations of disgust, úgh! yeùgh!, the sound of Scottish ch in lóch (which in Ireland is indeed spelt lóugh) - or mutated into the sound of [f] and 'ph'.
It is pronounced [f] in: tróugh, cóugh, Góugh, enoúgh, toúgh, roúgh, sloúgh skin (cf. slòugh swamp, and the English town Slòugh, both *slòu).
More often it is silent as in slòugh swamp - with quite a variety of preceding vowel sounds and spellings: ŏught, sŏught, bŏught, cåught, nåughty, Våughan, Våughn, dôugh, èight, nèigh, wèigh, slèigh ride (= slây kill), wèight heavy (= wâit time), frèight, heîght, bòugh, throûgh, thôugh, Búrrôughs, sîght, nîght, nîgh, and ough is even sometimes a schwa [ə] as in BrE bòrough, Scàrborough and thòrough, which in AmE are bòrôugh, Scàrborôugh, and thòrôugh, rhyming with fúrrôw. British English pronounces fürlôugh this way too.
gh uniquely sounds like [p] in híccoúgh (a variant spelling of híccup). In other words the digraph merely represents a hard g, whether Germanic, as in ghôst, ghoûl, ghāstly, or Italian, as in spaghéttì; and h serves to distinguish dínghy boat (which can have hard g or silent g, but always the ng sound) from díngy dirty (soft g: *dínjy).