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

Here is Why You Need An Adapter

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

Guam

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

Type A

Type B

Eritrea

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

Type C

Type L