Can a gerund be modified by an adjective?

Don't tell me.
Somebody has taught you that a noun formed from a verb by adding -ing is a gerund.
Indeed, that's what the "gerund" tag above says:

• Gerund: A noun formed from a verb by the addition of -ing.

Unfortunately, this is not true. A gerund is a verb, not a noun. And it behaves like a verb.
There are nouns formed from verbs with -ing -- as well as other things -- but they are not gerunds.
There are ways to check.

For instance, nouns can take articles and adjectives, while gerunds can take direct objects.
The writing in this book is atrocious. There's good writing and there's bad writing. (noun)
*The writing the novel exhausted her. *Good writing a novel is what you want. (gerund)

In the case of the first sentence, that's the noun writing, which can take an adjective.
In the case of the second, that's the gerund writing, which can take an adverb.