Answer:
It is a gerund because it is the -ing form of a verb that is functioning as a noun: it is the object of the verb enjoyed.
Step-by-step explanation:
On cold winter days, Mrs. Brown enjoyed snuggling under a blanket near the fire.
Mrs. Brown is the subject. Enjoyed in the verb. What did Mrs. Brown enjoy? Snuggling. So snuggling is the object of the verb.