What is a gerund phrase? How do you know it's a gerund phrase?

A gerund phrase will begin with a gerund it will also begin with ing word. also it will include other modifiers or other objects. Gerund phrases always function as nouns, so they will be subjects, subject complements, or objects in the sentence ...

Start with what a gerund is ... a gerund is a verb that has - ing at the end

ex: hearing, becoming, happening

so a gerund phrase is a phrase that includes the gerund and all of its modifiers and compliments

example: They like jumping on the trampoline

jumping is your gerund

They like jumping where? on the trampoline

so Jumping on the trampoline your gerund phrase