Make sure you're using a base Docker image that has GPU capabilities and using an environment that has a GPU.
If you're building your own custom Docker image, make sure you've added the following environment variables to make sure the Valohai execution can access the GPU.
ENV NVIDIA_VISIBLE_DEVICES all ENV NVIDIA_DRIVER_CAPABILITIES=compute,utility
You can also run
nvidia-smi in your step's command to see which GPUs can be found using the Docker image you're using for your execution.