Walking acts as a gerund here because it functions as a noun following the preposition ‘of’. A gerund is the -ing form of a verb used as a noun.

১. Present participles usually function as adjectives or part of continuous tenses.
২. A common noun is a general name for something, whereas ‘walking’ is an action turned into a concept.
৩. Gerunds are often called ‘verbal nouns’ because they represent an activity.

অতিরিক্ত তথ্য: Gerunds can serve as subjects, objects, or objects of prepositions.