I realize this reply is late, but when exiting Guam to HNL, you must go through US Immigration in Guam before you are allowed to access the departure gate. Upon arriving in HNL, you are required to pass through US Customs. The reason for this is that Guam participates in a Visa Waiver Program so you can travel from some countries to Guam without a visa, but are required to have a visa to travel from Guam to the US proper.