Douglas Harper's Etymology Dictionary
gu-
because g- followed by some vowels in English usually has a "soft" pronunciation, a silent -u- sometimes was inserted between the g- and the vowel in Middle English to signal hardness, especially in words from French; but this was not done with many Scandinavian words where hard "g" precedes a vowel ( gear (n.), get (v.), give, etc.). Germanic -w- generally became -gu- in words borrowed into Romance languages, but Old North French preserved the Frankish -w-, and English sometimes borrowed both forms, hence guarantee/ warranty, guard/ ward, etc.