Final answer:
In the sentence given, 'portraying' is the verbal that functions as a present participle, describing the subject 'Ruth Gordon'.
Step-by-step explanation:
The verbal in the sentence "In my opinion, Ruth Gordon is very effective at portraying a kooky grandma" is "portraying". This is a present participle, which is a form of a verb used as an adjective. It describes the manner in which Ruth Gordon acts. The subject of the sentence is "Ruth Gordon".