Here's why:
* The "e" at the end of the word is silent. This means the vowel sound is determined by the "u" alone.
* Short vowel sounds are typically found in closed syllables (syllables that end with a consonant). In "gesture", the "u" is in a closed syllable ("ges-ture").
Therefore, the "you" in "gesture" has a short vowel sound, similar to the "u" in "cup" or "fun".