You Do Need power plug adapter while traveling from Gibraltar to Belize.

Here is Why You Need An Adapter

The outlet and plug type used in Belize are A, B and G and in Gibraltar is G. This is how they look like.

Gibraltar

In Gibraltar, outlets of type G are in use.

Type G

Belize

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

Type A

Type B

Type G