you have three different options to get the conference by airplane:

Rome Fiumicino International Airport (FCO) 
From Fiumicino airport, you can reach the Rome Termini railway station by:
— Leonardo express train
— different bus lines
once you reached the Termini railway station you can continue to Gaeta by train (see below)
In the airport several  car  rental services are avalilable (the airport is abount 150km far from Gaeta):
— Airport car rental services

the route is available here
Rome Ciampino International Airpor (CIA)
From Ciampino airport, you can reach the centre of Rome by
— Trenitalia rail services
— different bus lines
Or you can hire a car (the airport is abount 120km far from Gaeta):
— Airport car rental

Naples International Airport (NAP)
The Naples railway station is few kilometers away from the airport and can be reached by buses or taxes.

By car

From Naples or Rome take the “A1” highway (autostrada, green road signs) and exit at Cassino, then turn right and take “SS Cassino-Formia” towards Formia-Gaeta.

Train icon

By train

From Rome or Naples there are hourly train services to the Formia-Gaeta train station.
Click on www.trenitalia.com to buy your tickets.
As departure station insert “Roma Termini” for Rome and  “Napoli Centrale”  for Naples.
As arrival station insert “Formia-Gaeta”

Taxi service (price: is about 20 euros) and buses (price: 1.5 euros) are available from Formia to Gaeta.

By bus

Local buses from Formia-Gaeta train station to Gaeta are available every 15 minutes (by COTRAL company).
Get off at the last stop (piazza Traniello).
Travel time is about 15 minutes