First, just in case you are not aware, the GAC (Guest Assistance Card) has been replaced with
DAS (Disability Access Service).
The 2 programs differ in how they work.
There is a thread near the top of this board about DAS for WDW and a separate one for
Disneyland. The first post of each thread is a FAQs about DAS.
The short answer to your question is, yes, you can use DAS for the viewing areas.
But, then there are some things about the viewing areas to be aware of:
1) some of the viewing areas are fairly small and fill up quickly, so getting there early is important. They usually rope them off an hour or more before the parade/Illuminations. And may be fairly well filled up soon after they open.
2) people with mobility devices are usually lined up very close together across the front row/rope/fence of the viewing area. One member if their party stands behind them and the rest of their party. The rest of your group may not be able to stand close by.
If it is very busy, there may be more than one row of guests with special needs
3) if the viewing area is deep enough, some people prefer to stand toward the back of it in order to have a buffer space between their group and the rest of the people. Not all viewing areas are deep enough for this.
4) once you are in the spot, at least some of your group needs to stay there. The person with a disability is usually not going to be able to be roaming around outside and there is usually not space to roam around in the viewing area.
5) most of the areas have no benches or other seating.