Though a guardian ad litem's report has great weight, information not available to the GAL or evidence that tends to rebut a GAL's opinion can sway the Court's decision.
Please note that this response is based upon the limited information available in the question. In addition, it is not intended to create an attorney/client relationship and is offered only as general information and not as legal advice.
Not always, but you should view the Guardian ad litem (GAL) as an important part of the process. If you have a lawyer, you should consult with your attorney about how best to cooperate and participate in the GAL process. Strategizing how to handle interaction with the GAL and ensuring he or she receives relevant information in a timely manner can have a big impact on your case.
This answer does not create an attorney-client relationship and is not intended to be legal advice.
Typically, a guardian ad litem's recommendation holds a good bit of weight, but not always enough to carry the day. It is very important to be organized, focused, and cooperative in your interactions with guardians ad litem. Make sure that you focus on your children's needs and not on everything that you think your ex has done wrong to you or in life in general. If you have references or documentation that supports the information that you want the guardian to know, make it easy for the guardian to have access to people or items who can support your point of view. Some guardians ad litem are very good at diligently doing their jobs and giving insight to the court. Many are not. It is important that you are prepared to present your case to the judge, regardless of what the guardian ad litem thinks or recommends. If you have a guardian whom you think is biased against you, it is often easier to have a meeting with the guardian and try to sway her or him than it is to have the guardian removed. If that does not work, focus on presenting your case to the judge. It is ultimately up to the judge and not the guardian to make determinations in custody and visitation cases. If you make a strong enough case, many judges will disregard the recommendations of the guardian ad litem.
Cooperating and communicating with the Guardian ad Litem ("GAL") is important in preparing for a contested hearing in a custody case. It is important to understand not only the GAL's position, but also the basis for his or her opinion and what he or she thinks is important. I have represented clients who prevailed against the GAL's opinion because the GAL was emphasizing factors that were less important to the judge and discounting factors that were important to the judge. For instance, a GAL in one of my cases valued continuity and stability in custody, relationships with step-siblings, a nicer home and school district, while the judge gave more weight to the primary relationship with the parent seeking custody and the denials of visitation rights by the other parent.
In short, the GALs are not perfect, they are not the judges in these cases, and your attorney should be prepared to put on evidence contradicting the basis for the GAL's opinion and values should you disagree with it.
The foregoing is intended to be general legal information. You should consult with a Virginia attorney to discuss all your options in light of your particular circumstances.