Here's why:
* Schwa sound: The schwa sound is a very short, unstressed vowel sound represented by the symbol /ə/. It's often described as the sound in the word "about" or "the".
* "Gallon" pronunciation: In the word "gallon," the vowel sound in the first syllable is a long "a" sound, as in "father". It's not a short, unstressed sound like a schwa.
Therefore, "gallon" doesn't have a schwa sound.