Here's why:
* Gerunds are verb forms ending in "-ing" that function as nouns.
* Possessive pronouns show ownership or possession.
Example:
* His running was impressive. (The running belongs to him)
In this example, "running" is the gerund, and "his" is the possessive pronoun indicating that the running is something that belongs to him.
Here are some other examples:
* Her singing was beautiful.
* Their arriving late was a disappointment.
* My studying helped me pass the exam.