What guardian ad litem means
A guardian ad litem, often shortened to GAL, is someone the court appoints to represent the child's interests in a case, distinct from either parent's interests. Depending on the jurisdiction the role may be filled by an attorney or a trained volunteer, and their exact duties vary, but they commonly investigate, interview both parents and sometimes the child, and report or make recommendations to the judge.
Because a GAL gathers information independently and the court often gives their view real weight, how you present your side matters. Organized, factual documentation gives the GAL something concrete to work with and supports the impression that you are focused on the child rather than on the conflict.
Example: when a GAL asks about your involvement in the child's schooling, being able to reference dated entries about parent-teacher meetings, homework routines, and appointments is more convincing than general claims of being involved.
The powers and title of this role differ by state, and some places use terms like child's attorney or best-interest attorney, so ask your attorney what applies in your case.