If they're native to the island they're called Chamorro, however if they're immigrants they're just Guamanians.
I asked my sister in law, shes a Guam native.
Or you can also call them, Guamibears : )
Living/residing on Guam: GUAMANIAN
Native people of Guam: CHAMORRO
A resident of Guam, an unincorporated territory of the United States in the western Pacific Ocean, is called a Guamanian.
A person from Venezuela is a Venezuelan.
A Russian
friend
Hermit!
a gypsy!
Cantabrigian
friend
A "Duluthian"
haligonian
Police Call - 1954 Guam was released on: USA: 1955
Friend
A person who lives in the same city as you is commonly referred to as a "fellow resident" or simply a "neighbor."