Those coming by car can arrive either from Rome or Naples by the A1 motorway, exit Cassino, then take the provincial road to Formia and finally Gaeta.
If you don't want to take the motorway coming from Rome you can take the Appia or the Pontina.
From Naples, on the other hand, take the Domiziana and pass through Formia. From Caserta you take the Appia and pass through Formia.

By train, get off at the Formia-Gaeta stop, then take a bus or taxi.