She opened the door and for a moment he stood looking at her.
Transformed so the gerund phase as subject can be seen:
- |Looking at her| is what he had been doing for the last ten minutes.
A gerund phrase can be the subject or object in a sentence.
Or a subject phrase or object phrase complement.
gerunds as objects and subjects
A gerund is a verbal that ends in -ing and functions as a noun. The term verbal indicates that a gerund, like the other two kinds of verbals, is based on a verb and therefore expresses action or a state of being. However, since a gerund functions as a noun, it occupies some positions in a sentence that a noun ordinarily would, for example: subject, direct object, subject complement, and object of preposition.
stand as used above is intransitive, so, "looking at her" can be said to a subject complement. Verbs used to describe body positions are often followed by a subject complement like that or like the ones below The subject complements are gerundial phrases.
- They sat staring at the moon.
- She lay dozing in the hammock.
- He knelt praying for hours.
Introductory participial phrases:
- Walking the dog, she breathed the fresh air.
Those are generally set off by commas, yes. However, usually, they serve to clarify or strengthen the main clause. I would prefer to see something like this:
- Enjoying the fresh air, she walked the dog for two hours.
There, breathing the fresh air describes her, whereas "walking the dog" doesn't seem to. It's grammatically fine, but semantically a bit odd.
introductory participial phrase