In order to be a GAL, you need to meet specific requirements set forth by the Probate Court. There are two kinds or levels of GALs: Evaluators ("E" list)and Investigators ("F" list).
To apply for the "E" list, you must be licensed to practice medicine or psychology, or a licensed independent clinical social worker, a licensed marriage and family therapist, a licensed rehabilitation counselor or a licensed mental health counselor. See http://www.mass.gov/courts/courtsandjudges/courts/probateandfamilycourt/applicationcateevaluator.pdf
To apply for the "F" list, you must be licensed to practice medicine or psychology; a licensed social worker, certified social worker, or independent clinical social worker; a licensed marriage and family therapist; a licensed rehabilitation counselor; or a licensed mental health counselor. Get form at: http://www.mass.gov/courts/courtsandjudges/courts/probateandfamilycourt/applicationcatfclinician.pdf
Attorneys should complete this form: http://www.mass.gov/courts/courtsandjudges/courts/probateandfamilycourt/applicationcatflawyer.pdf