You are new! (I see you have only one post). Welcome to disABILITIES.
GAC stands for Guest Assistance Card.
Most queques at WDW are accessible for wheelchairs and/or scooters (also called
ecvs) to assist people with these mobility devices. Just having a wheelchair or
ecv allows you to use the accessible queues and accessible entrances.
But, some people need to use these entrances, but don't have a wheelchair, so they have an invisible disability that requires them to use that access. Sometimes children don't have a wheelchair, but will need to stay in their stroller in the queque. Some people have totally different needs, like needing to be in the front for shows because of a vision disability or in a specific area to be able to use devices for those with hearing impairments. Some people need a quieter place to wait because of conditions like autism, or to wait where they will be protected from the sun. The GAC is a way to let the CMs know about the assistance needed.
If someone in your party has these needs, they can request a GAC at Guest Services at any of the parks. You need to be able to explain what your needs are and the CM there decides whether or not you are eligible for a GAC. The GAC is usually valid for your entire stay for the person with a disability and up to 5 other members of your party.