Unfortunately, scholarly consensus is that *Decretum Gelasianum *is a later forgery, since it purports to be dated 381 yet quotes Augustine writing in 418.
You are misunderstanding the issue. This is a red herring frequently used by polemicists. Also the site itself is bogus, it states that it is about the writings of Tertullian, but he lived around 160, nearly 200 years before Rome I. So it starts off in error and continues in error. Additionally it needs to be noted that It is The Catholic Church that has declared that the decretal is a forgery, However that has nothing to do with the Canon of the Council. The historicity of the 382 CE Council of Rome is well documented and no credible scholar doubts it.
Even so, several of councils, starting with Hippo and Carthage have declared the same Canon down through the mellenia.
Here are the subsequent councils reaffirming the Canon of Pope Damasus I:
**Council of Hippo (393) **
North African Church council in union with and under the authority of the Bishop of Rome approved a list of OT and NT canon (same as later approved by the Council of Trent)
**Council of Carthage (397) **
Another council in union with and under the authority of the Bishop of Rome approved a list of OT and NT canon.
**Pope Innocent I, Bishop of Rome, 401-417 (405) **
Responded to a request by Exuperius, Bishop of Toulouse, with a list of canonical books of Scripture; this list was the same as later approved by the Council of Trent.
**Second Council of Carthage (419) **
THis Council in union with and under the authority of the Bishop of Rome approved the same list of OT and NT canon.
The Council of Nicea (787)
The Council of Nicea II in 787 ratified the same canon as authoritative for the Eastern Churches.
Council of Florence, an ecumenical council (1441)
Same complete list of OT and NT as the previous Councils.
Trent, of course, reaffirmed that Canon that The Church has held since 382 CE.