In which sentence is the underlined word a gerund? even the microscopic bug was a tiny, breathing creature. dolphins are breathing only when they are above the water. breathing is a basic function controlled by the brain.

Breathing in the first sentence is a participle acting as an adjective.

The second sentence is an auxiliary verb. is and breathing make up the verb.

Breathing is ... go no further. Breathing is the subject of the third sentence. It can only be a noun or a pronoun when it is a subject. So here it is a gerund.