If you went through a formal court proceeding to get the guardianship, I would check with the court that issued the guardianship papers (in Georgia, it's the county probate court). They should be willing to provide you with those papers for a reasonable fee.
If it was not a formal guardianship done through a court, but instead your child signed papers to give you custody of your grandchild, I would check with the attorney who prepared the paperwork. Hope this helps. Enjoy your trip to Spain!