One main public lot handles almost all parking at GUM
Antonio B. Won Pat International basically runs off a single public car park in front of the Main terminal, so anything labeled “Overflow Parking” is likely just extra space opened from the same on-site area when things get tight. There’s no separate gate, shuttle, or branding the way you’d see at bigger Pacific hubs; airport maps and the official Guam Airport site list only this one public lot for passengers.
Parking here is set up more like a long-term surface lot than a fancy garage, and the airport explicitly markets it as public parking for Main terminal departures and arrivals. All the publicly available info groups short- and long-stay in the same zone in front of the building, so any overflow use would still mean the same walking route: a short walk straight across the access road into the check-in area for flights like UA to Honolulu or Manila.
No separate pricing sheet appears online for anything called Overflow Parking; all rates roll up under the general Guam Airport public parking fees published on the airport’s ground transportation page. That means you should budget off those standard daily and multi-day numbers, not expect a discount or premium for an overflow area, and plan to pay the same at the pay machines as you would if you’d parked in the regular rows closest to the Main terminal doors.
Because there’s no dedicated shuttle service listed for any overflow area at GUM, parking further out just adds minutes on foot instead of adding a bus ride. In Guam’s heat and humidity, that short extra walk can feel longer, especially if you land on an afternoon arrival bank and the apron temperature is running into the 30s °C. Good news: the path from the far end of the public lot to the terminal doors still stays sidewalk-only, with no need to cross high-speed traffic.
If the front rows of the public lot are packed, keep driving slowly toward the outer edges before you give up and start circling off-airport; the airport’s own information points you back to that same on-site public parking as the default solution, even during busy holiday periods.