The GPU is already used for the model forward and badckward passes.
However, since your model is really small, the CPU workload might be the bottleneck and would thus cause a low GPU utilization.
E.g. using some imaginary numbers:

GPU forward + backward pass takes 1s
data loading and processing as well as accuracy calculation takes 10s on the CPU

In this case, the CPU workload would be the bottleneck.