A little explanation first:
GAC stands for Guest Assistance card. It is not meant to (and usually won't) shorten your wait in line, but will give a more appropriate place/condition to wait in. It is basically a way for someone with invisible disabilities to quickly tell CMs what sorts of accomidation they need. Someone who has needs that can be taken care of by having a wheelchair or
ecv in line does not need a GAC. Someone with other or additional needs might benefit from a GAC.
You go to Guest Relations in any of the parks (the person with a disability needs to be with you) to request a GAC. Explain your needs/difficulties to the CM there. GACS are usually issued for the person with a disability and up to 5 additional members of their party. Usually it will be issued for the entire length of your trip. You don't need to get one in each park; the same GAC is good in all the WDW parks.
The general GAC has instructions on it, then the CM adds a stamp (or sometimes prints out on a computer) with additional instructions. One stamp allows a stroller to be used as a wheelchair. That means it can be brought into all the buildings and used in lines just like a wheelchair. That means you would be able to take it into shows and she could stay in it for shows (although in many cases, she will be able to see better if she is not in the stroller since strollers are usually sort of low). For rides, you would be able to take it right up to the boarding area. There are some rides where wheelchair users can stay right in their wheelchairs. There is usually a longer wait for the wheelchair car. Stroller users (unless they have a special needs stroller that is designed to be tied down in schoolbuses or vans) don't have a way to secure their stroller into the wheelchair cars for rides like the AK Safari and would not be allowed to stay in the stroller. Some rides (like Pooh and Buzz Lightyear) have wheelchair cars where the wheelchair is not really tied down, but they are not designed for strollers - sit too low and the wheelchair user sits all by themself in front of the other passengers. So, she would not be able to stay in her stroller for any rides, but possibly may for some shows.
For some rides, you will be able to fold up your stroller and take it with you. For other rides, you'll just leave your stroller where you board the ride and pick it up when you get off.