Not necessarily. There are different stamps on different GAC's depending on the guest's needs, and not every ride's fast pass line will necessarily serve that guest the best way. Like other posters have said, there are, often times, lines for guests with mobility disabilities that aren't always obvious. While, yes, having a GAC or wheelchair may get you to the front of some lines, it may not do the same for other rides, depending on how they train those cast members to deal with guests with disabilities.
True, but a vast majority of the GACs (upwards of 95% I'm willing to bet, and I'm sure there are other attraction CMs on here who would agree with that number) include the "Alternate Entrance" stamp, which is the one that uses the fastpass lines at attractions with fastpasses. If you get a GAC at either MK or Studios, ALL GACs are pre-stamped with the "Alternate Entrance." Only Epcot and DAK still dare to give out the other stamps on their own, and even then, once the guest realize it's not the all-day FP they were expecting (especially when they see another family with the "same" red card who ARE being admitted to the FP lines immediately), they usually go back to Guest Relations and pitch a big enough fit to get the "Alternate Entrance." Or they just learn the trick of going to the "right" park's GR.
The worst thing is that there are an unsettling number of people who get the "Alternate Entrance" stamped GACs, but then they demand that they get the green light stamp instead, which is reserved for GKTW only. I don't know what the perceived benefit is - there is very little difference between the way the two are treated at most attractions (maybe they would get on faster at TSMM or Soarin' or something? Honestly, every attraction I have ever worked, the two stamps are given the exact same treatment). I feel like those who want the green light stamps are the ones faking it - the ones with the entitlement mentality of I-am-special-and-deserve-immediate-gratification-in-all-things-always-everywhere.