You need an ETA for Guam if you have a Hong Kong SAR passport

Travellers need a travel authorization to visit Guam for Business or Tourism. The Guam and the Commonwealth of the Northern Mariana Islands Electronic Travel Authorization (GCNMI ETA) is an online application that determines the eligibility of visitors to enter Guam. Once the application is approved, travellers receive their GCNMI ETA confirmation electronically. A valid GCNMI ETA is required to visit the country. If the GCNMI ETA is expired or doesn’t match the traveller’s passport information, travellers will not be able to board their flight. A GCNMI ETA is electronically linked to a passport and valid for 2 years. If travellers receive a new passport, they must apply for a new GCNMI ETA. United States permanent residents or holders of valid United States visas don’t need a GCNMI ETA. Travellers must have a Hong Kong Special Administrative Region passport with a Hong Kong identification card or a British National Overseas passport with a Hong Kong identification card.