You Do Need power plug adapter while traveling from China to Guam.

Here is Why You Need An Adapter

The outlet and plug type used in Guam are A and B and in China are A, C and I. This is how they look like.

China

In China, outlets of type A, C and I are in use.

Type A

Type C

Type I

Guam

In Guam, outlets of type A and B are in use.

Type A

Type B