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

Here is Why You Need An Adapter

The outlet and plug type used in Guam are A and B and in Eritrea are C and L. This is how they look like.

Eritrea

In Eritrea, outlets of type C and L are in use.

Type C

Type L

Guam

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

Type A

Type B