The admin of this site gives an accurate description here: http://www.spainexpat.com/spain/information/the_gestor_gestoria/
A gestor is not a licensed professional. In reality, he is just a clerk with experience and good contacts. In theory, you could open a gestor?a tomorrow, so you need to be selective when you search one out. He can help you with basic administrative processes, but if you can find a gestor?a that doubles as an abogado or contable or some such within the same office, then you can be more confident.
To find a good gestor is like finding a good solicitor. Word of mouth recommendations are useful. Ask people who are in business, such as your local pub, or shop. See if you can find one who can speak your language well. (And I mean, well. Not just ?can get by?). Many gestor?as are multi-lingual. You will find them listed in paginasamarillas (yellow pages, in case you weren?t sure) under Gestor?as Administrativas.
REPUTATION is key!