Please, give us instructions for GPU solo mining

You would want to use either sgminer or ccminer depending on your brand of GPU (AMD or NVIDIA), then instead of pointing your GPU to the pool you'd be pointing it to your local Denarius wallet. You would need to setup a .conf file similar to usual solo mining for Denarius and then point the GPU to like -o
http://127.0.0.1:32339 -u denariusrpcuser -p denariusrpcpass
Its almost exactly the same as solo mining with a CPU, just different software.