When I was going but could not do stairs I just told the CM and they never asked for a Gac. It might have been because I always go off season or they could tell by looking that I would have a problem.
I don't remember all the rides with stairs but the official WDW site should have a disability page with that info?
What I can think of are;
Splash Mountain
Toy Story
Big Thunder Mountain Railroad
WDW railroad
The treehouse
sorry all I can think of right now.
The WDW site doesnt have a list of attractions with stairs (would be nice if they had more information there though).
At MK - these have stairs:
Splash Mountain - there is an accessible bypass just before the stairs that sends guests to a waiting area near the exit
WDW Railroad at Frontierland and Main Street - there is a ramp at both stations, but it is steep and winding
Swiss Family Robinson Treehouse - stairs are winding, steep and narrow
Big Thunder Mountain RR has no stairs, but has some very steep ramps, which I think people remember as stairs.
At Epcot - these have stairs:
American Adventure - stairs to get to the 2nd floor for the show. By the stairs, there is an escalator, so you can avoid the stairs
O Canada - there are stairs to reach the 2nd floor level shop area and the entrance to the movie. You can avoid that without a GAC by following the path thru the garden on the right side of Canada as you face the shops from the front of Canada
At DHS - these have stairs:
Beauty and the Beast show is in a theater with stairs to get down to the seats. You can sit in the rear to avoid the stairs.
Toy Story Mania - has stairs just after the point where guests pick up their 3D glasses. There is a ramp that bypasses the stairs.
Lights, Motors, Action - stairs in the amphitheater to get to the seating.
Indiana Jones - stairs in the amphitheater to go down to the seating. You can sit in one of the back rows.
Fantasmic - stairs in the amphitheater to go down to the seating. Very back row is for wheelchairs and
ECVs. Several rows ahead are for guests with special needs.
For Animal Kingdom - these have stairs
Dinosaur - stairs in the boarding area. Guests who cant do the stairs use an elevator just to the left as you leave the preshow area.
Festival of the Lion King - theater has stadium seating with some seats up many stairs. Much of the ground level seating is for guests with special needs.
Nemo - stairs in theater to get down to seating or sit in stadium seating in the very rear of the show. Back row is for guests with wheelchairs and ECVs, but you can sit in the front row of the stadium seating.
This is all I can think of offhand. There may be more, but I need to get to bed.