Some people with DS (I'm assuming Down Syndrome) might have needs that a GAC would help with, others don't. The GAC is given based on needs, not diagnosis. She may not actually have any needs that a GAC would be helpful for.
Start out with reading the GAC information in the disABILITIES FAQs thread.
That is a summary of GAC information from DIS posters experience and information from CMs.
The Guidebooks that said that a GAC allows guests with the card to be ushered to specified entrances for the attractions is not really correct. That makes it sound like there is one GAC that always allows the same things to be done for everyone who has one. It also makes it sound like people with disabilities are always ushered to specified entrances, which is also not correct.
In actual fact, there are many different GACs that allow different things. Each GAC has a message stamped on it that is appropriate for the needs of the person the GAC was issued to. Exactly how it works depends on the needs of the person/what is stamped on the GAC and what is available to meet those needs at the attraction.
For example, for some children, being able to stay in their stroller might be a reasonable solution to their needs. If that is what their GAC allows, they would be able to stay in it in the regular line and have it treated the same as a wheelchair (sometimes they use a sticker for the stroller).
You should have a better idea of how GACs work after reading the GAC information in the disABILITIES FAQs thread.
NOTE TO POSTERS: PLEASE DON'T POST THE SPECIFIC WORDING ON GACS OR SPECIFIC INFORMATION ON WHAT THAT WORDING ALLOWS.