"Which sentence?" you ask. I'm confused. You only provided one sentence, and that sentence <em><u>does</u></em> have a gerund functioning as an object of a preposition!
They thanked us exuberantly for meeting with them to discuss the deal.
Explanation:
Did you intend to ask someone to identify the gerund that is functioning as the object of a preposition??? If so, then .....
prep. gerund
They thanked us exuberantly for <em><u>meeting</u></em> with them to discuss the deal.
