70C is fine for a GPU underload. Typically, when a GPU usage is low it is because the CPU is bottllenecking it. What are your system specs and what games are you running when you see the 50% usage? What resolution and detail level are you playing at? The lower the resolution and detail level, the lower the GPU usage.
If you are playing a CPU heavy game, such as BF1 on a subpar CPU, you are going to have low GPU usage.