A gerund phrase always begins with a gerund. A gerund is a word that ends with –ing. A gerund phrase will always contain modifiers. The gerund phrase works in a sentence as a noun, but they are verbs ending with –ing, which operate as nouns.  Thus, as functioning nouns, they can be the subject, the subject complements or the object part of the sentence structure.
What are some examples of gerunds? Writing, walking, driving, thinking, studying, cooking.
So we know the rules of a gerund phrase, that it is a phrase beginning with the gerund, is contains modifiers or other object words, it contains a verb ending in –ing which acts as a noun. Note that the gerund phrase is not a sentence beginning with a gerund, but a phrase.  It can be found anywhere in a sentence.

examples of the gerund phrase

Hitting the ball really hard with the tennis racket was a great de-stressor.
The gerund phrase is at the beginning of the sentence, and is the subject of the verb ‘was.’

He grew into a charmingly pleasant young man.
The gerund phrase is at the end of the sentence, and the object of the verb ‘grew.’

Following the baseball game, the group drove to a nearby restaurant
Following the baseball game is the subject of the verb ‘drove’.

The gerund phrase works in a sentence as a noun, but they are verbs ending with –ing, which operate as nouns.  Thus, as functioning nouns, they can be the subject, the subject complements or the object part of the sentence structure. It is easy to recognize in a gerund phrase in a sentence because they have a very distinctive function with simple rules.