Here's why:
* Past participle: A past participle is a form of a verb that is used to form perfect tenses (e.g., present perfect, past perfect) and passive voice.
* Gerund: A gerund is a verb form that ends in "-ing" and acts as a noun.
Example:
* Past participle: "She had reacted calmly to the news." (part of the past perfect tense)
* Gerund: "Reacting to the news was difficult." (acting as the subject of the sentence)
Since you haven't provided a sentence, it's impossible to say definitively whether "reacted" is being used as a past participle or a gerund.
To determine its function, look at the sentence structure:
* Is it part of a perfect tense? (e.g., has reacted, had reacted) If so, it's a past participle.
* Is it acting as a noun? (e.g., reacting is important) If so, it's a gerund.