Final answer:
A gerund is an -ing verb that functions as a noun, and when combined with modifiers or an object, it is known as a gerund phrase. These can serve as subjects, like 'Volunteering' in the provided example, or take on other roles in a sentence to add descriptive detail to experiences.
Step-by-step explanation:
The grammatical function of the gerund is as a noun. A gerund is any -ing verb that functions as a noun in a sentence. In the given sentence, 'Volunteering' is the gerund and it functions as the subject of the sentence. It represents the action of volunteering and serves as the main focus of the sentence.