You Do Need power plug adapter while traveling from Guatemala to Guinea-Bissau.

Here is Why You Need An Adapter

The outlet and plug type used in Guinea-Bissau is C and in Guatemala are A and B. This is how they look like.

Guatemala

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

Type A

Type B

Guinea-Bissau

In Guinea-Bissau, outlets of type C are in use.

Type C