Is the word gaggle a pronoun?

No, the Word 'gaggle' is a noun, a Word for a group of geese (while they are on the ground); a Word for a disorderly group of people; a Word for a thing.

A pronoun is a Word that takes the place of a noun in a sentence.

The pronoun that takes the place of the noun 'gaggle' is it.

Example: A gaggle of photographers waited by the exit. It sprung into action with flashbulbs and shouts when the door opened.