The Deum is probably the best-known piece by Langlais. It uses fragments of this gregorian hymn in a majestic and triumphal way. It's typical Langlais in his gregorian-triumphal manner.
There are many other fine organ pieces on this hymn (Attaingnant, Buxtehude, Demessieux, Dupré, Robert Anderson, Naji Hakim, Tournemire, etc, etc) but Langlais version is probably my prefered one.
The sampleset, Aristide n°1, is a composite set which was elaborated to be a clone of St_Ouen de Rouen. It's IMHO (only), a beautiful achievement faithful to the original (big roaring swell, enlighted by the carillon, sheer power, chamade s not overwhelming but giving attack to the tutti, fine 32', classical mixtures present but non assertive...). I discovered it by a topic on the Hauptwerk forum here :
What is quite fun is that Thorsten (Thways) has uploaded today a big Vierne on another CODM emulating St-Ouen. So, you will be able to compare the sounds of these two sets.