The combination of letters -gu- sometimes records or represents a difficulty experienced by writers who had been educated in French. After the Norman Conquest, the administration of England was largely in the hands of such men, and the records largely kept by their scribes. Because the sound systems of French and of English (and then of Old French and Old English) are different, the new scribes used characteristic combinations of letters to represent the new sounds they heard. One example of this is the 'w-' of English. When this precedes an '-i-', it is not infrequently shown as 'gu-' by French scribes. Examples include such pairs as 'Guillaume', the French equivalent of 'William'; 'guile' and 'wile'; 'guise' and the archaic 'wise'.
Before '-a-', the French scribes already used 'gu-' in writing their own language to represent a consonant + vowel pairing which came, over time, to lose the vowel in speech. The written French language followed this phonetic development while English didn't; hence modern French writes garder where English writes 'guard', and garantie where English has 'guarantee'
More usual in modern English are near synonyms such as guard and ward; in current English, there are several pairs in which two words of very similar meanings have two separate spellings - and sounds. The spelling with gu- maintains a 'hard g' sound; that with 'w-' represents the loss, in speech, of the initial consonant in the consonant + vowel pairing 'g- + '-u-'. This appears to have originated in different dialects of early modern French. In this way, the words 'reward' and the archaic 'guerdon' are cognate, as are the verbs guage and wage;and a local name in the south east of England for a 'guillemot' (bird) is 'willock'. (The etymology of the English name is the familiar abbreviation for William 'Will' + diminutive '-ock', as in 'hillock'; the French has the fuller Guillaume + a diminutive suffix -ot. (In the form of English spoken in Wales, the bird is known locally as guillem, from the Welsh form of the same name, Gwilym.) The English word 'wicket' (gate) is known as a 'guichet' in French.