What is the question, precisely, I got a bit confused...?
Is it: "Do we need GPAW installed to compute optical properties?" The answer would be no, you can compute optical properties with ATK.
Or: "We need GPAW to compute optical properties. How to install it?" The answer is: pls refer to the GPAW documentation, GPAW is not a code we officially support. That said, it's possible to install GPAW inside ATK, but it takes a bit of effort.
