You Do Need power plug adapter while traveling from Guinea to Tonga.

Here is Why You Need An Adapter

The outlet and plug type used in Tonga is I and in Guinea are C, F and K. This is how they look like.

Guinea

In Guinea, outlets of type C, F and K are in use.

Type C

Type F

Type K

Tonga

In Tonga, outlets of type I are in use.

Type I