As we all know, this type of music is modal, not tonal in nature. Therefore, some things which you read about tonal harmony can't be applicable here. Often the problems arise when we want to use D-T chordal progressions. Today I would like to share with you my personal recommendations which I hope you will find useful.
Here is how you can harmonize the melodies of Gregorian chant:
1. Figure out the mode of the chant.
2. Compare it with the major or minor by finding out which scale degrees are different.
3. Harmonize the chant as you would in major or minor but in chords which have different scale degrees don't use foreign accidentals.
4. This way your cadences will have a harmony which works with your particular mode.
5. If the phrase ends on II-I in the soprano and the mode is similar to major but has a lowered leading tone, then you can play a minor dominant chord and a tonic.
6. If in the same mode the phrase ends on VII-I in the soprano, then you can play the first inversion of the chord of the seventh scale degree and the tonic.
7. If the mode is similar to minor but has a lowered second scale degree and the phrase ends on II-I, then you can play the root position chord of the seventh scale degree and the tonic.
8. If in the same mode the phrase ends on VII-I, you can play the first inversion of the chord of the seventh scale degree and the tonic. Likewise, you can figure out other chords from the natural tones of the mode.
Try this approach today for yourself and share your experience in the comments.