You Do Need power plug adapter while traveling from Dominica to Guatemala.

Here is Why You Need An Adapter

The outlet and plug type used in Guatemala are A and B and in Dominica are D and G. This is how they look like.

Dominica

In Dominica, outlets of type D and G are in use.

Type D

Type G

Guatemala

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

Type A

Type B