Hello,
To the best of my knowledge, there is no punctum mora positioning in gregorio 6.0.0. Adding zeroes and ones works for horizontal and vertical episemata only.
You can always "hack" the .gtex file by changing (in this example) \GrePunctumMora{8}{0}{0}{0} to \GrePunctumMora{7}{0}{0}{0}, but the hack will be overwritten next time the autocompile sets in (e.g. next time you modify the gabc file).
Also this punctum mora placement just looks bad. I understand it was forced by the (very theoretical) interdiction made to Solesmes to change the spacing from the Vaticana, but at the point we're at, this kind of legislation is out the window.
In Christo,
Matthias