How are you using Gregorio? The only option on Source and Summit would be to change the size of the initial.
It's a bit wonky for me. The default point size is too big with my typeface of choice, but using \grechangestyle{initial} with \fontsize and 32pt text works well, except for Magnificat antiphons. Putting "Ant. ad Magnif." causes this even after changing the size of the letter, but it's not necessarily the case with all descending characters. Your mileage may vary (I'm not sure if it's font-related tbh). I once forgot the period in "Magnif.", it worked correctly, then it didn't once I fixed it… and I removed it again, only for this behavior to return, so I adjusted annotationraise as I normally do with this case.
In my case, raising the annotation 0.03mm is more than enough to correct the problem without disturbing the look of the score too much; hanging the size of the initial (another option) is a little to obvious, but again YMMV. (I know, the antiphonale tends to label each antiphon, but for booklets, an extra thing not attached to the score = more widows).