OpenCL is an API, just like OpenGL. There may be open source or closed source implementations in the future.
The GPL doesn't prohibit linking to closed source libraries that are more or less part of your OS, or we couldn't calll the NVidia OpenGL implementation from Tremulous. So using OpenCL should be possible.
However OpenCL is not a Physics engine, it is an interface to the raw compute power contained in current GPUs. When you want a Physics simulation, then you have to implement all the math yourself.
I think before you ask people about which engine should be used, you should ask what this engine should do within the game. Only physically correct animation of some map elements ? Or physically correct movement and collisions ?