Passport validity
The source states no minimum passport validity.
Visa rules · Tourist entry
Entry rules for Guam from its official sources, and every passport we have a page for.
Guam admits visitors under the US Visa Waiver Program or the Guam-CNMI Visa Waiver Program. Visitors eligible for neither need a visa from a US embassy or consulate.
Official sources
The source states no minimum passport validity.
Pick your passport for the full rule, its source quote and the check date.