Here's why:
* Gerunds are verb forms ending in "-ing" that act as nouns. They can be the subject of a sentence, the object of a verb, or the object of a preposition.
* "Jumping" can function as a noun. For example:
* Subject: Jumping is good exercise.
* Object of a verb: I enjoy jumping rope.
* Object of a preposition: I'm tired from all the jumping.
Therefore, "jumping" is a gerund in these examples.