For a plain and ordinary Medical Certificate to say
that you were too sick to work on <whatever date> ?
You do.
It is uncommon for a GP to charge you for this.
Other certificates (such as the one for returning to work after a Workers Comp related injury,
or a vocational medical report, such as if you need to be examined for a health standard
that you are required to meet before being allowed to work)... getting those is different.