Here's why:
* Collective nouns refer to a group of individuals. In this case, a "gaggle" specifically refers to a group of geese.
* Verb is a word that describes an action. "Gaggle" is not an action.
* Adjective modifies a noun. While you might say "a gaggle of geese", "gaggle" itself is not describing a noun.
* Common noun is a general name for a person, place, or thing. While "geese" is a common noun, "gaggle" is a specific term for a group of geese.
Let me know if you have any other words you'd like to categorize!