This sound is made up from a few factors, Firstly it’s written in phrygian mode.
For those that are unsure of what phrygian mode is i will explain it here;
Modes are just different types of scales, so a standard major scale (i.e C D E F G A B) is also the Ionian Mode, what separates the different modes are the intervals between each note of the scale.
For example the natural minor of C Major is A Minor (A B C D E F G), this can also be referred to as the Aeolian mode.
Phrygian in relation to the major key will have several notes flattened and will look like this (C Db Eb F G Ab Bb),
However i think it’s easier to remember that phrygian is very close to the natural minor except for one note, the second of the minor scale is flattened making the new scale look like this A Bb C D E F G.
Alright back to the Gothic choir,
Here I’ve made a small snippet of a choir in action, looking at the melody line you can see it’s in phrygian (A minor also using Bb), the shadows under each of the notes are diatonic but don’t stay with the chord outlined in the bass, the movement between chord I and VII are what gives the choir that flavour that we’re looking for.
The second major point to keep in mind is that because of the movement in the melody it’s important to keep the Bass Peddled, this will help the music feel a bit more stable while throwing out the harmonics that create dissonance with chord VII in the melody line, It also stops the chord changes above feeling like a function rather than a melodic device which is what we’re aiming for.
Here is what it sounds like!