First of all, "U" has no sound here, but it has a function. "G" as 2 sounds in French, soft "j" if followed by the vowels "e" or "i" and hard "g" if followed with the vowels "a", "o", "u". So if "guerre" was spelled "gerre" we would say something like "jer", not "ger". Secondly, we don't pronounce much final "E" in French, unless they have an accent. If it was spelled "guerré" for example, we would say "ger-ray". So tha's why. We dodge the U and the final E.

This is not the spell but the pronunciation https://fr.wiktionary.org/wiki/guerre It sounds like fer, mère, air A ne pas confondre avec guère (= peu) - La guerre viendra bientôt et il n'y a guère d'espoir