Of course, the scales of Gregorian chant permitted the B-flat, so the music implicitly had major and minor modes, though theorists didn't always treat them separately.

Some further, slightly off-topic points: I think the Greek names weren't always used. Some authors, I think, used numbers, e.g. "modus primus", "modus deuterus", etc. And I think not all the music in the chant repertoire conforms strictly to the theoretical system.

I think the horizontal and vertical tritones were avoided, for the reasons mentioned by the theorists quoted by Margo Shulter: It's hard for some singers to sing (even three successive whole tones can be tricky--I know a choirmaster who had to rehearse his choir repeatedly on a passage with three successive whole-tones; some of the singers kept wanting to flatten the last one) and it has a distinctive sound which a music-writer might not want to introduce into the music.

T.