You Do Need power plug adapter while traveling from Gabon to Swaziland.

Here is Why You Need An Adapter

The outlet and plug type used in Swaziland is M and in Gabon is C. This is how they look like.

Gabon

In Gabon, outlets of type C are in use.

Type C

Swaziland

In Swaziland, outlets of type M are in use.

Type M