Case law on Immunity for GAL's, yes they are accountable

Minority Rule: No Immunity 

In Fleming v. Asbill, 42 F.3d 886 (4th Cir. 1994) (applying South Carolina law), the court recognized that a cause of action for malpractice negligence may be asserted against a guardian ad litem. There, the court, relying on Simpson v. Doggett, 159 S.C. 294, 156 S.E.771 (1930), stated that South Carolina common law makes it clear that a guardian ad litem is liable to his ward for the negligent performance of his duties. Moreover, S.C. Code Ann. § 20-7-127 provides that a lay guardian ad litem may be held answerable to his ward for gross negligence. If a lay volunteer can be liable for gross negligence, then a paid professional can be held liable for negligence as well. 

A similar result was reached in Marquez v. Presbyterian Hospital, 159 Misc. 2d 617, 608 N.Y.S.2d 1012 (Sup. Ct. 1994). In that case, the court held that where a guardian ad litem acts in his or her capacity as an arm of the court, "the proper standard where there are very young children, and the guardian ad litem role predominates, is that liability should attach only if there is a showing that the law guardian failed to act in good faith in exercising discretion or failed to exercise any discretion at all." 608 N.Y.S.2d at 1018. A guardian ad litem, might, therefore, be held accountable in negligence while acting within the quasi-judicial function.