2^4 finite field multiplication in VHDL
"optimise" ? no : improve, yes`.

I would do two things :  


If you can cleanly express the formula in a VHDL function, I would initialise the gfarray from that function. It'll be easier to maintain and less error-prone than manually transcribing every value. As it's a constant, the function won't be executed at runtime.
Minor : I'd declare the input ports as natural range 0 to 15 and clean up a lot of unnecessary type conversions - if permitted.