Regulator of gamma-secretase activity, which specifically activates the production of amyloid-beta protein (amyloid-beta protein 40 and amyloid-beta protein 42), without affecting the cleavage of other gamma-secretase targets such has Notch. The gamma-secretase complex is an endoprotease complex that catalyzes the intramembrane cleavage of integral membrane proteins such as Notch receptors and APP (amyloid-beta precursor protein). Specifically promotes the gamma-cleavage of APP CTF-alpha (also named APP-CTF) by the gamma-secretase complex to generate amyloid-beta, while it reduces the epsilon-cleavage of APP CTF-alpha, leading to a low production of AICD. Interacts with APP; specifically interacts with the CTF-alpha product of APP. Interacts with the gamma-secretase complex (By similarity). The protein is first synthesized as a holoprotein form of 98 kDa and rapidly processed into the gamma-secretase-activating protein 16 kDa C-terminal form, which constitutes the predominant form. Belongs to the GSAP family. Extended N-terminus.