(always appears along with manic elation, ignorance, laziness, shamelessness and lack of faith)

What is gloominess? It is the way in which the mind cannot function properly and is associated with listlessness [gti mug]. Its function is to aid all basic and proximate emotions.
It is a very subjective tendency in which physical and mental heaviness and sluggisheness dominate.

Concerning this, the Ratnamala states,

Gloominess is any lack of activity
Due to heaviness of body and mind.
The Abhidharmakosavyakhya states,
What is gloominess? It is heaviness of body and heaviness of mind. It is a state of physical inertness and mental inalertness.
The lam-rim explains it in the same manner.

The statement that its function is to aid all of the basic and proximate emotions means that all of the emotions increase in intensity on account of gloominess. Therefore, the Adhyasayasamcodanasutra states,

Whoever rejoices in sleep and sluggishness
Will have a distorted view of his world,
As when body fluids, air and bilious fluids
Become excessively present in the body.

Whoever rejoices in sluggishness and sleep will be dulled
Just as food which has turned bad is not healthy.
When the body is heavy, the complexion will be unhealthy,
And even one's speech will become incoherent.

Whoever rejoices in sluggishness and sleep
Will be dull and take no interest in the dharma
And will shy away from all virtuous qualities.
And since brightness leaves his life, he will remain in darkness.