The GPU utilization depends on the workload. E.g. if your model is quite small you might see a low GPU utilization throughout your training. On the other hand you might see some peak utilization if e.g. the data loading is a bottleneck.
Could you share some code so that we can have a look at possible issues?
As a small side note: Variables are deprecated since PyTorch 0.4.0, so you can just use torch.tensors in newer versions.