A gerund phrase is a phrase consisting of a gerund and any modifiers or objects associated with it. A gerund is a noun made from a verb root plus ing (a present participle). A whole gerund phrase functions in a sentence just like a noun, and can act as a subject, an object, or a predicate nominative.
If you look up the definition of gerund (pronounced JER-und), you will find that it means “an English noun formed from a verb by adding -ing”; that is, a present participle used as a noun.