Please note that Mommsen uses the AUC chronology (Ab Urbe Condita), i.e. from the founding of the City of Rome. You can use this reference table to have the B.C. dates
The Roman main army in its turn advanced along the Allier down
into the canton of the Arverni. Vercingetorix attempted to prevent
it from crossing to the left bank of the Allier, but Caesar
overreached him and after some days stood before the Arvernian
capital Gergovia.(48)
48. This place has been sought on a rising ground which is still
named Gergoie, a league to the south of the Arvernian capital
Nemetum, the modern Clermont; and both the remains of rude
fortress-walls brought to light in excavations there, and
the tradition of the name which is traced in documents up to the tenth
century, leave no room for doubt as to the correctness of this
determination of the locality. Moreover it accords, as with
the other statements of Caesar, so especially with the fact that he
pretty clearly indicates Gergovia as the chief place of the Arverni
(vii. 4). We shall have accordingly to assume, that the Arvernians
after their defeat were compelled to transfer their settlement from
Gergovia to the neighbouring less strong Nemetum.