Here's why:
* Gerunds are verb forms ending in "-ing" that function as nouns. They can be the subject of a sentence, the object of a verb, or the object of a preposition.
* Participles are also verb forms ending in "-ing," but they function as adjectives. They describe nouns or pronouns.
* Verb phrases are groups of words that act as verbs. They include the main verb and any helping verbs.
In the sentence "Sketching is a fun hobby," "sketching" is the subject of the sentence, making it a noun and therefore a gerund.