Consider the following Old English words: gān ‘gone’, clāþ ‘cloth’, brād ‘broad’. They belonged to the same lexical set as OE gāt ‘goat’, and we would expect them to have evolved like the rest of the GOAT set, since they do not share any characteristic subregularities with any recognised “minority flock”. Even the spelling of gone and broad (similar to that used in stone and goat, respectively) suggests that they were still members of the GOAT set at the time when the modern orthographic conventions were becoming fixed. And yet they have parted company with other words containing OE ā. Broad has joined the CAUGHT set (with Modern English /ɔː/, as in cause), while the other two vary between CAUGHT and LOT (Modern English /ɒ/ or its unrounded counterpart /ɑ/ as in American dialects). Note also that while OE sc(e)ān ‘shone’ yields the expected outcome /ʃoʊn/ in America, the normative British pronunciation is /ʃɒn/, with a shortened vowel.
Such cases are truly irregular and call for individual explanation. We know that the shortening of the vowel of clāþ cannot date back to Old English (OE “claþ” would have become Modern English “clath”). OE ā produced a mid-low rounded vowel /ɔː/ (conventionally spelt ǭ to distinguish it from other O spellings) after the Norman Conquest, during the Middle English period. Indeed, the word was very often spelt clothe, clooth or cloothe in Middle English, apparently indicating a long-vowel pronunciation. Note that the OE plural clāþas has normally developed into Mod.E clothes, with /oʊ/ (the th may be mute, but that is another story). Today, however, clothes is no longer regarded as the plural of cloth, but rather as an independent collective noun (a case of word duplication!). The distribution of the modern pronunciations of cloth points to an early shorthening of Middle English /ɔː/, as a result of which the word joined the LOT set. Then, in some (but not all) mainstream accents of Modern English, the short vowel was affected by the lengthening heard in moss, cost, lost, frost, moth, often, off, cough, etc., induced by the following voiceless fricative.
The development of broad must have been different, since the word does not show a short vowel in any major accent, and the final consonant is not a voiceless fricative. When the Great Vowel Shift of the 15th century transformed ME /ɔː/ into Early Modern English /oː/ (diphthongised to /oʊ/ in most contemporary varieties of English), one stray sheep left the flock as its vowel underwent an irregular lowering (for reasons that elude us). That lowered pronunciation merged with the new /ɔː/ that resulted from the smoothing of the diphthong /aʊ/ after the Great Vowel Shift (in such words as daughter, caught, law, cause, and drawn).
|Gonna be gone|
Perhaps there was another sheep of the same contrary disposition, since the long vowel of gone in the accents that rhyme it with drawn is best explained in the same way. Why do we find /gɒn/ ~ /gɑn/ as well? It’s hard to say at which historical stage the shortened variant originated. It could have appeared before the Great Vowel Shift, immediately after it, or still later, with the same result. It is quite possible that it has arisen many times. It is worth observing that high-frequency verbs often display irregular phonetic simplification, possibly because sloppy pronunciations are easier to tolerate in words more or less predictable from the context. Note the similarly unexpected short vowel of says and said, does and done, as well as been (pronounced like bin in American English). Been, said, does, done, says, and gone (in that order) are all among the 500 most frequently occurring English word-forms.
I will return to this interesting correlation between frequency of use and erratic behaviour (which usually consists in some kind of phonological erosion – the shortening, reduction or loss of speech segments).