MAGMA does not implement a multi-GPU gemm. It is easy to fashion a multi-GPU gemm out of single-GPU gemm, but it depends on how the matrices are distributed across the GPUs. See this previous message:http://icl.cs.utk.edu/magma/forum/viewtopic.php?f=2&t=569&p=1784#p1784
If you have a specific data distribution in mind for A, B, C, I can help you figure out what to do.
CUBLAS has a CPU interface that uses multi-GPUs.https://developer.nvidia.com/cublasxt