You Do Need power plug adapter while traveling from Grenada to French Guiana.

Here is Why You Need An Adapter

The outlet and plug type used in French Guiana are C, D and E and in Grenada is G. This is how they look like.

Grenada

In Grenada, outlets of type G are in use.

Type G

French Guiana

In French Guiana, outlets of type C, D and E are in use.

Type C

Type D

Type E