During a hearing in my pending child custody/visitation action, opposing counsel stated to the Judge that I "had previously assaulted" both my fiancé and the GAL; and, in fact, the GAL felt she needed a deputy to escort her to her car after a hearing."
The answer to your question depends in what capacity the attorney allegedly made the statements that you found offensive or untrue.
Generally, testimony under oath in a court of law is considered protected speech. The words spoken by an attorney as a case is litigated before a jurist in a legal proceeding are also protected speech. Furthermore, an attorney can proceed upon the information and belief of his or her client, or witnesses the proceeding. The wisdom behind such protection has to do with the greater benefit to all of society and encouraging witnesses and participants to freely participate in trials to insure that justice is served for all.
In Virginia, where I live, that is a Class 1 misdemeanor with jail time of up to 12 month. Opposing Council knew that I had not assaulted my fiancé and any charges brought against me had already been nolle pros in the domestic relations court.
The fact that a prosecutor decided NOT to charge you after an arrest, doesn't negate the fact that you were arrested for allegedly committing an offense against the peace and dignity of the state, and/or against the person of the complainant/victim.
My question is: can I sue him criminally and/or civilly For slander because he said this in open court for the whole gallery to hear?
You, as can any other person, are free to sue any person. You need simply complete a few forms, pay a fee (or file as a pauper), and you're on your way along the path to justice.
Generally, slander cases can't be brought for testimony given under oath in a court of law.
That said, YES, you are still free to bring such a lawsuit.
You, me, anyone will always get his or her day in court.