What version of MATLAB are you using?
In R2010b-R2012a releases, the GPU object was named parallel.gpu.GPUArray. In those releases you would create an array of all zeros on the GPU using:
z = parallel.gpu.GPUArray.zeros(8192,1);
In R2012b, the GPU object was renamed to gpuArray, so in R2012b and onward you would create the same array using this line of code:
z = gpuArray.zeros(8192,1);