San Gregorio Taumaturgo was a small early 18th century oratory of a confraternity, which used to be in what was the Via Pedacchia. Most of this lost street is now under the Via del Teatro di Marcello to the west of Il Vittoriano near Piazza Venezia. This is in the rione Campitelli.
It features on the Nolli map of 1748, and in 1758 the city highway authority set up a long inscription opposite which mentioned the oratory and which basically prohibited people practising sex and excretion in the locality. Before 1870, the citizens used to urinate and defecate in the streets as a matter of course.
The Confraternita di San Gregorio Taumaturgo owned the oratory, and used to meet here until about 1815. They then took over the church of Santa Chiara, but did not maintain it properly and allowed the roof to fall in. They then moved to Santa Maria dei Miracoli. Meanwhile, the oratory was taken over by the Compagnia dei Santi Marco Evangelisto e Marco Papa, which was dedicated to veneration of the Blessed Sacrament and was based at San Marco.
They, in turn, abandoned the oratory in the middle of the 19th century and it was deconsecrated. Before its demolition, it was an taverna or drinking establishment for what Armellini calls the "lowest class" of people.
The Via Pedacchia was a mediaeval street which started from the south-east corner of the Piazza di San Marco, then ran south, south-west and west to the former piazza with fountain which is now in the Via di San Venanzio. It was destroyed in the building of Il Vittoriano in the late 1880's.
The oratory was on the east side of the portion that ran south, and the site is now the semi-circular piece of grass in front of a fountain on the west hand side of Il Vittoriano, near the road.
The plan was rectangular, with a triumphal arch dividing the nave from the presbyterium. Armellini asserts that it retained some features of an oratory even when profaned; he mentions the doorway as having a "sacred character" and there being a pair of blocked-up windows flanking it.
When he wrote this in 1891, the building was still standing but must have been demolished very soon after. The nearby oratory of Santa Maria del Carmine e Sant'Antonio had already been demolished.