First of all; welcome to the board!
Have you read the sticky on the top of the DISability boards? It gives a lot of great info.
What a GAC is? It stands for Guest Assistance Card. What it does? It is a Card that explains to CM's working rides etc. what type of Assistance a Guest needs so the CM can accomodate where possible, no more, no less.
WDW incourages people that have stamina problems to use a wheelchair or
ECV, there is no help to be expected on that front from a GAC. You should ask yourself one question; what need might you have that are not met by standard services/not clearly visable for a CM to see?
If you've got an (or more) answer to that question? Go to Guest Services and explain and they'll take it from there. For instance, some people can't wait in the sun. They can get a GAC explaining this, so they can wait in a shaded area. Or for small children that need a wheelchair due to health reasons, but because of their age/height/weight parents using a stroller as a wc. They'll get a GAC stating they use the stroller as a wc, so they can bring it along in the lines for the kid.
Just to stress it again; the GAC is NOT a front in the line pass. Depending on your needs, you'll wait in line just like everybody else and might find you actually wait longer (for instance because you need an accessible vehicle and there are only very little of them, increasing the waittime for them)
If your needs are visable, for instance because you use a wheelchair (that would about scream 'can't walk for mile and miles'), you don't need a GAC for those types of needs. The CM will notice it and if needed sent you to the right spot if a ride doesn't have wheelchair accessible lines.
So, as you can see, a GAC really has very little to do with state of health, but has all to do with what needs a person has that are not visable.