You don't specify which version of AE do you have ?
did you update the nvidia driver / cuda driver ?
did you update your AE version ? I can be needed for some recent GPU to be well recognized,
if not, you may have to edit a text file,
here's a thread form another forum with similar problem for a GTX 770 GPU
and maybe a screen capture of the GPU info window in AE would help
Manually add it to the raytracer_supported_cards.txt file or enable the option for unsupported cards in the prefs depending on which version of AE you actually use. Still, no guarantees that this will make it work... AE is way too picky about these things for its own good...
i've tried editing the text file on my mac.. but it didnt work!
What, exact, version of AE are you using?
Did you know the GPU doesn't do much in AE? It mostly just accelerates the obsolete ray-traced renderer? See this: GPU (CUDA, OpenGL) features in After Effects
I've never used the ray-traced renderer, there are much better ways to create 3d objects in AE. Thus the death of the ray-traced renderer.
Now, some third-party plugins like Element, Red Giant Universe, etc. do make use of the GPU, but they don't require AE to recognize it at all.