A GAL is a guardian ad litem and is the voice of the child in court. A custody evaluator is a professional in the field of mental health who is qualified to perform a child custody evaluation and make a report to the Court. There is a Florida statute that deals with custody evaluations. Check Avvo Legal Guides on the subject.
Just to add to the previous answer, the guardian ad litem represents "the best interest" of the child, which means the he or she observes, talks to all involved, reviews documentation, etc., and based on all the information gathered, makes a recommendation to the court as to what is in the child's "best interest." The guardian ad litem can speak to the child and relay the child's wishes to the court, but does not actually represent the child. The guardian ad litem can be a lay person or an attorney who has been trained to perform their duties and prepare reports in accordance with the Program's standards. The guardian ad litem prepares a report to the court, but the report is based on hearsay and is usually not admitted into evidence as a result. However, the guardian ad litem's testimony can still be presented.