There are still several ways to do this. If you want to support older devices, you should stick with the OpenGL ES 2.0 shaders. There are GPUImage and ogles_gpgpu frameworks for this. The latter even supports Android systems.
I also wrote a master's thesis on this topic: Parallel computing for processing digital signals on graphic devices of mobile devices .
Now you can do more complex GPGPU stuff with OpenGL ES 3.0. For example, this post . Apple now also has the Metal API, which allows you to perform even more computing operations on the GPU. Both OpenGL ES 3.x and Metal are only supported by newer devices with the A7 chip.