Question

A verb form that functions as a noun is called a(n) .

Answer 1

It's called an adjective.  Adjectives are "grammatically related" to a noun to describe or modify it. 

Answer 2

Gerund

Occasionally, a verb can function as a noun if “ing” is added to the end of it.  As you may know, a noun can perform or receive an action, and when an “ing” is added to the end of a verb, this is possible.  For instance, write is a verb is in I write where “I” is the subject, and “write” is the verb.  However, if you add “ing” to “write” as in “writing,” it becomes a noun as it can therefore perform or receive the action as in Writing makes me happy or I like writing, respectively.