You cannot run a Mex function using gpuArray/arrayfun. In fact, this is specified in the help for gpuArray/arrayfun:
FUN must be a handle to a function that is written in the MATLAB
language (i.e., not a built-in function or a mex function).
I am a little bit confused by what you are trying to do. Does the SPAMS toolbox provide GPU support that you are trying to access in your Mex file? I do not know anything about that product, but a quick web search did not reveal any indication that the SPAMS software was intended to run on a GPU.