As written, it is not a gerund, it is a participle. It would be a gerund if the "Sarah" were changed to "Sarah's".
Student or Learner
In the following sentence:
"I don't see an advantage in Sarah being promoted."
why should we use "gerund" on the mentioned sentence?!
because being promoted is subject of gerund or Sarah who is being promoted or because of "in" as preposition we use continues form?!
With all respect to your source, I disagree. A gerund always functions as a noun. In you r sentence there is no place for a noun after Sarah. If her name was changed to a possessive adjective, then the noun would work.
Jane doesn't like John drinking on the weekend. In this case, the -ing word is a participle that describes John.
Jane doesn't like John's drinking on the weekend. In this case, the -ing word is a noun and John's is an adjective.
No, but when people use object pronouns, they're usually still thinking of it as a gerund.