Taken from Duden it is not an adverb but in this case a pronoun. (Indefinitpronomen).

Some are some are not inflected. This is not.

The adjective 'genügend' would be treated as any adjective:

Es gab nicht genug Gründe.

vs

Es gibt keine genügenden Gründe.

Note that it kann also be used as a particle (Partikel):

Der Schrank ist groß genug

Taken from Duden it is not an adverb but in this case a pronoun. (Indefinitpronomen).

Some are some are not inflected. This is not.

The adjective 'genügend' would be treated as any adjective:

Es gab nicht genug Gründe.

vs

Es gibt keine genügenden Gründe.

Note that it can also be used as a particle (Partikel):

Der Schrank ist groß genug

Taken from Duden it is not an adverb but in this case a pronoun. (Indefinitpronomen).

Some are some are not inflected. This is not.

The adjective 'genügend' would be treated an any adjective:

Es gab nicht genug Gründe.

vs

Es gibt keine genügenden Gründe.

Note that it kann also be used as a particle (Partikel):

Der Schrank ist groß genug

Taken from Duden it is not an adverb but in this case a pronoun. (Indefinitpronomen).

Some are some are not inflected. This is not.

The adjective 'genügend' would be treated as any adjective:

Es gab nicht genug Gründe.

vs

Es gibt keine genügenden Gründe.

Note that it kann also be used as a particle (Partikel):

Der Schrank ist groß genug