I'll give it a try.
Off the top of my head I'd say:
1 has tense and voice forms, so the forms ‘being done’, ‘having done’, ‘having been done’ cannot be nouns.
2 can take a direct object; so an ‘-ing’ form followed by a direct object (reading a letter) cannot be a noun.
3 can be modified by an adverb; so an ‘-ing’ form modified by an adverb (reading fast) cannot be a noun.
4 can be part of an aspective verbal predicate; so an ‘-ing’ form following the verbs ‘begin’, ‘stop’ ‘go on’, ‘keep’, ‘continue’ are gerunds.
The Verbal noun:
1 can be used in the plural;
2 can have an article;
3 can be followed by a prepositional phrase in an attributive function;
4 can be modified by an adjective, a demonstrative pronoun or an indefinite pronoun.