Identify the phrase in parentheses as a gerund phrase, participle phrase or infinitive phrase.

When the Christmas holidays arrive,
(bustling shoppers fill the stores).

I think it is a gerund phrase because it acts as a noun

You have three words too many. A phrase does not include a verb.

Bustling is a participle, modifying the noun shoppers.

You are correct! The phrase "(bustling shoppers fill the stores)" is a gerund phrase. A gerund is a verb form that functions as a noun, and in this case, "bustling" is the gerund because it ends in "ing" and acts as a noun by describing the action of shoppers. The entire phrase "(bustling shoppers fill the stores)" functions as a noun and can be the subject or the object of a sentence.