Gravissima is an organ stop

Organ stop

An organ stop is a component of a pipe organ that admits pressurized air to a set of organ pipes. Its name comes from the fact that stops can be used selectively by the organist; some can be "on" , while others can be "off" .The term can also refer...

 that is usually pitched at 64'. It is usually a resultant

Resultant (organ)

In pipe organs, a resultant tone is the sound of a combination of organ pipes that allows the listener to perceive a lower pitch.This is done by having two pipes, one pipe of the note being played, and another harmonically related, typically at its fifth, being sounded at the same time...

(a 32' stop combined with a 21 2/3' stop, which is a fifth, producing a difference tone of 8 Hz on low C.) It is also usually the lowest stop in the organ that has one.

Tonal quality

This is usually a quiet stop, producing a deep rumble, often used to create a profound effect in music.