Question

Is planting a garden a gerund phrase

Answer 1

Well, a gerund phrase begins with a gerund (an "ing" ending word), and will include other modifiers or objects. They always function as nouns, too!
"Planting a garden" starts with a -ing gerund (planting), and "a garden" is the object. Does "Is planting a garden" a full sentence? No, it's not, but it is a gerund with an object, so it can be considered a gerund phrase- just not a full sentence one.