McGillop was a scientist working for UNIT who was present at the National Gallery when the Zygons gained access to the Black Archive.
McGillop was at the National Gallery along with Kate Stewart and Osgood when the Eleventh Doctor and Clara Oswald arrived to view the painting Gallifrey Falls No More. As the Doctor left the room he received a phone call from a future version of the Eleventh Doctor telling him to verify the number and to move the painting to the Black Archive.
Later, while talking with Osgood he was accosted by a Zygon who took his form. This Zygon and its comrades were stopped in the Black Archive when the Eleventh, Tenth, and War Doctors entered the room through the painting McGillop moved there earlier.
Along with Kate Stewart, he conducted peace talks with the Zygons. (TV: The Day of the Doctor)