A gerund phrase consists of a gerund (a verb ending in -ing that functions as a noun) and any modifiers or complements that go along with it.
Among the options provided, "Walking uphill" is the only one that can be used as a gerund phrase. It includes the gerund "walking" and the modifier "uphill."
The other options do not function as gerund phrases:
- "Over the rainbow" is a prepositional phrase.
- "To inspire" is an infinitive phrase.
- "The author of three books" is a noun phrase.
So the correct answer is Walking uphill.