The closest airport to Tumon Bay is Tamuning Airport, which is located in Tamuning. If you plan on visiting or staying in this area, landing at Tamuning Airport will be your best bet.
The best prices found on momondo for flights to Guam are:
$103 per person with T'way Air
$122 per person with Air Seoul
$135 per person with Jin Air
Guam is open for travel. Travelers arriving in Guam are subject to quarantine or self-isolation depending on where they are traveling from. Travelers from high-risk COVID-19 areas should present a negative PCR test for COVID-19 from within 72 hours of their arrival. More details are available here. If you are planning to travel to Guam at this time, it is recommended that you stay up to date on current restrictions and follow proper safety measures while in public.