Rufinus: The Master inserted the arguments of this question here but gives the solution in Case 16.

Gratian C.16 q.3 d.p.c.15:  St Gall 673, fol. 136:

Potest quod aliter intelligi prescriptio illa longi temporis qua negat Gelasius ecclesiarum statum posse mutari.  Nam longum tempus intelligitur decennium vel vicennium quemadmodum et ex legibus habetur.  Huius longi temporis prescriptio ecclesiis obici non potest, set sola prescriptio xxx. annorum.

Prescription of a long time can be otherwise understood from that which Pope Gelasius denies when he says that the state of the church cannot be changed. A long time is understood as ten or twenty years and is established by <Roman> law.  This prescription cannot be used against the Church, but a prescription of thirty years can be used against the Church.

Köln, Erzbischöfliche Diözesan- und Dombibliothek 129, fol.  185r

Köln, Erzbischöfliche Diözesan- und Dombibliothek 129, fol.  186r