I think I heard the PS5 is sharing its main memory between CPU and GPU just like a laptop, and an answer on quora.com seems to say the same.
So, supposing that guy on quora and I were right, then theoretically the GPU can address 16G of RAM. But practically the operating system and the game need space too, (and that same guy says the OS takes 4 G), so now you have max 12G RAM left. Now subtract what the game itself needs and THAT is effectively the RAM the GPU has left.