"제가 친구를 도와주는 것을 좋아해요."
Translation:I like helping my friends.
Naturally when I think of friends in general I would say I like helping any or all of them, so I assume friend is plural. If I wanted to specify a particular friend, I might use the topic marker <sub>은/</sub>는, thereby putting emphasis on a particular friend.
Korean is vague like this, and context has to supply the rest of the story, where cultural assumptions can mean a lot in the spoken language, more than in American English.
Actually "I" or "제가" is the subject, because the action of liking is performed by "I" or "제가". "Helping" or "도와주는" is still a gerund, simply as the object of "like" or "좋아해요". But yeah, the tip of replacing the gerund with a pronoun works really for checking if it actually is a pronoun.