Gerunds are not nouns. Gerund phrases are noun phrases, but that is quite different. Since gerunds are not nouns, but are instead verbs, if they are modified, the modifier must be an adverb. Since there are nouns derived from verbs in English by adding the suffix -ing to a verb, the situation is confusing, but these nouns in -ing are not gerunds.
In your example
"The skillful defusing of bombs is XXXX"
"defusing" is a noun derived from the verb "defuse", and we can tell that "defusing" is a noun by its being preceded by an article, "the", by its being modified by the adjective "skillful", and by the fact that the logical direct object "bombs" has to be made into a prepositional phrase with "of".
Although this is not an example of a gerund, there is a gerund form of this:
"Skillfully defusing bombs is XXXX"
where the gerund "defusing" is shown to be a verb by the facts that it is not preceded by an article, it is modified by an adverb rather than an adjective, and it takes a direct object (which nouns do not do).