MMEDIATELY AFTER the ALLELUIA by Guerrero, Dr. Horst Buchholz improvises in a marvelous way, based on the “Tu Es Petrus” plainsong theme:
We usually add a short organ piece after the Gospel, when the Subdeacon carries the Evangeliarium to be kissed by the celebrant, who is then incensed, walks to the pulpit, dons his biretta, and so forth.
I don’t know if this is written down “officially” in the rubrics, but the sacred liturgy has always been considered something living. For the record, Fr. Adrian Fortecue allows—in his sensational handwritten book of instructions—organ music at the same place shown in the video:
Fortescue doesn’t want it after the Gospel, probably because he never had Solemn High Mass at his parish church.