Most publishers (Music and otherwise) will engrave the music and save the file as a pdf and then drop it into the master file that is Adobe's Indesign (or Quark Xpress).
Having engraved music editions (including hymnals), I will engrave the music without margins (the score/staff) it has no left margin or right margin, taking into account the size of the master .indd file. So if the master is set on a 6 x 9 hymnal, and the margins are appropriately set (~.44in--we use pica for spacing), then the engrave score is only 5.125 in. wide. The length, depending on the header and footer is default to 7.25, but always changed so there is minimal margins (.15 in at top, and .25 in at bottom). Title headings, and extra stanzas and other verbiage (such as hymn tune, metre, credits) are set using Indesign text blocks.
So, Gregorio can generate files without margins that can be incorporated into the master file. Which is what I would recommend--one has way more control over all aspects of the look and feel.
Hope this helps.
Jeffrey A. Trimble
Organist
St. Joseph Parish Community
79 Case Ave