After Effects will take advantage of the GPU for some things automatically.
You have to tell After Effects manually to use the OpenGL renderer for final rendering and for previews. Most people don't, and for good reason. The OpenGL renderer is fast, but it doesn't give results that are as good as the CPU renderer.
I use the OpenGL - Interactive preview mode.
The GPU isn't very important for After Effects.
You can do your own searches to get answers like this. For example searching for OpenGL pulls up this information.
As Todd said, you shouldn't render with OpenGL at all, you'll have bad surprises. AE doesn't take much advantage of Graphic Card, but some third party plugin does.
ok!!! thanks for the answers guys! u really helped me!