It can be and is, technically, but it’s a little complicated. It depends on which version or versions of the Dies Irae you are talking about. The Dies Irae is the liturgy of a Requiem Mass for the Dead from the medieval Catholic Church. But it inspired many composers, like Mozart and his Dies Irae, and many others wouldn’t technically be Gregorian Chants. Here’s the entry on Requiems from wikipedia where it explains:
”The Requiem Mass is notable for the large number of musical compositions that it has inspired, including the settings of Mozart, Verdi, DuruflĂ© and FaurĂ©. Originally, such compositions were meant to be performed in liturgical service, with monophonic chant. Eventually the dramatic character of the text began to appeal to composers to an extent that they made the requiem a genre of its own, and the compositions of composers such as Verdi are essentially concert pieces rather than liturgical works.”
Hopefully someone who knows more about music and liturgical history of the Catholic Church than I do will weigh with more, possible better, information. :-)