The Gerund

a) has tense and voice forms; so the forms being done, having done, having been done cannot be nouns;

b) can take a direct object; so an "-ing" form followed by a direct object (reading a letter) cannot be a noun;

c) can be modified by an adverb; so an "-ing" form modified by an adverb (reading fast) cannot be a noun;

d) can be part of an aspective verbal predicate; so "-ing" forms following the verbs to begin, to stop, to go on, to keep, to continue are mostly gerunds.

THE VERBAL NOUN

a) can be used in the plural:

Memorize the proverbs and sayings.

b) can have an article:

the banging of the door;

c) can be followed by a prepositional phrase in an attributive function:

the clicking of the clock;

d) can be modified by an adjective, a demonstrative pronoun or an indefinite pronoun.