Many Mass of St. Gregory images include pictorial details from the Passion narratives in the background. In this case, the details are quite numerous:
On the left side we see the ladder used to take Jesus' body down from the cross. Inside the rungs are the spear that opened the wound in Jesus' chest and the sponge used to taunt him with vinegar. Left of the ladder are the nails, the nail extractor, and a sword, perhaps to indicate the dividing of Jesus' robe. Right of the ladder are the heads of a woman and a man, a fist, a tied length of cloth, and some ointment jars. There are a total of three jars, corresponding to the three women in Luke 24:1-12 who brought spices to the tomb on the Sunday morning.
Hanging from the right crossbar is a cowled tunic. Directly beneath it are Veronica's veil, a hammer, a scourge, and the third ointment jar. Right of these is the pillar where Jesus was scourged and another scourge. The rooster that Peter heard after denying Jesus stands atop the pillar. On the far right are the water with which Pilate washed his hands, the kiss of Judas, an urn (?), and Caiaphas. The latter is identified by his headdress and humeral, which also appear in images of the high priest officiating at Jesus' circumcision.
For a detail of the central panel, with commentary, please follow this link.
Read more about images of St. Gregory the Great.